fix(api): a refused write is named, in the reader's language

Deep-review round 5, over round 4's own fixes.

- `notifyActionError` preferred the server's `detail` over the caller's translated fallback, and
  `reportFailure` deliberately leaves 403/404/429 to the caller — so for exactly those statuses the
  dozen call sites round 4 converted started printing whatever English a route hand-wrote into an
  otherwise Hungarian UI ("No user with that email.", "Not subscribed to this channel", "Unknown
  job"). A STRUCTURED detail is still preferred: `req` has already run it through `localizeDetail`,
  so it is both localized and more specific. A plain string is not.

- The other half of the same defect, on the backend: round 4's `get_or_404(what="channel")` sweep
  reached exactly ONE of the five identical sites in channels.py and shipped the HU/EN string with
  it, so the Block button said it in Hungarian and the four controls beside it in English. All five
  are converted, and the subscription / share-recipient / scheduler-job 404s with them (two new
  reasons in both locales).

- `PlexCollectionEditor`'s add/remove, take-over and delete were try/finally with no catch — in the
  very file round 4 edited to give `createNew` one. 403 and 404 are the caller's to report, so the
  tick simply never appeared and a CONFIRMED "delete collection" did nothing, silently.

- `UploadDialog` is the only `uploadReq` caller and kept an empty catch whose comment still claimed
  the global error modal had it. It had not since `reportFailure` became shared: a proxy 502 raised
  only the self-clearing connection notice and a 413/403/404 nothing at all, so the dialog snapped
  back to its pre-upload state with no error named. The user's own Cancel stays silent — and since
  a cancel and a network drop both carry status 0, that difference is now a named `isAborted`
  rather than a string literal at each end.

Regression tests for each (5 vitest, 2 pytest, plus two existing assertions corrected), all
mutation-checked: 6 mutations, each red on the assertion it is named after. vitest 1007,
pytest 1127.

backend/app/routes/channels.py

@@ -26,7 +26,7 @@ from app.models import (
     User,
     Video,
 )
-from app.routes._common import get_or_404, owned_or_404
+from app.routes._common import get_or_404, not_found, owned_or_404
 from app.sync.explore import explore_ingest_page
 from app.sync.runner import backfill_channel_now, run_recent_backfill
 from app.sync.subscriptions import apply_channel_details
@@ -287,9 +287,7 @@ def channel_detail(
     needed — a stub, or a channel last enriched before those fields existed (published_at is a
     reliable sentinel: every real channel has a creation date). The enrich uses the API key
     (1 unit, cached after), so it's skipped for the quota-less demo account."""
-    channel = db.get(Channel, channel_id)
-    if channel is None:
-        raise HTTPException(status_code=404, detail="Unknown channel")
+    channel = get_or_404(db, Channel, channel_id, what="channel")
     if channel.published_at is None and not user.is_demo:
         try:
             with (
@@ -366,9 +364,7 @@ def explore_channel(
     it, then ingests one page of its uploads (newest-first) marked via_explore + enriched.
     Returns the next page token so the page can "load more". Guarded against the shared
     backfill quota reserve; blocked for the demo account (it spends shared quota)."""
-    channel = db.get(Channel, channel_id)
-    if channel is None:
-        raise HTTPException(status_code=404, detail="Unknown channel")
+    channel = get_or_404(db, Channel, channel_id, what="channel")
     if _is_blocked(db, user, channel_id):
         raise HTTPException(status_code=403, detail="You've blocked this channel.")
     if quota.backfill_reserve_reached():
@@ -426,7 +422,7 @@ def _user_subscription(db: Session, user: User, channel_id: str) -> Subscription
         )
     ).scalar_one_or_none()
     if sub is None:
-        raise HTTPException(status_code=404, detail="Not subscribed to this channel")
+        raise not_found("subscription")
     return sub
 
 
@@ -512,9 +508,7 @@ def reset_backfill(
     state (a "reset" trigger). Clears the channel's backfill markers, opts it back into deep
     backfill, and re-runs its recent pull immediately; the deep scheduler re-pages the full
     back-catalog on its next run. Idempotent — videos upsert by id, so nothing duplicates."""
-    channel = db.get(Channel, channel_id)
-    if channel is None:
-        raise HTTPException(status_code=404, detail="Unknown channel")
+    channel = get_or_404(db, Channel, channel_id, what="channel")
     channel.backfill_done = False
     channel.backfill_cursor = None
     channel.recent_synced_at = None
@@ -544,9 +538,7 @@ def subscribe(
     local subscription with YouTube's returned resource id and enriches the channel's metadata
     if it's still a stub, so the new row renders properly in the manager. It does NOT pull the
     channel's videos — the scheduler picks those up on its next run, like any subscribed channel."""
-    channel = db.get(Channel, channel_id)
-    if channel is None:
-        raise HTTPException(status_code=404, detail="Unknown channel")
+    channel = get_or_404(db, Channel, channel_id, what="channel")
     existing = db.execute(
         select(Subscription).where(
             Subscription.user_id == user.id, Subscription.channel_id == channel_id

backend/app/routes/downloads.py

@@ -38,7 +38,7 @@ from app.models import (
     MediaAsset,
     User,
 )
-from app.routes._common import get_or_404
+from app.routes._common import get_or_404, not_found
 from app.security import hash_password
 from app.userscope import is_messageable_user, messageable_clauses
 from app.utils import iso
@@ -802,7 +802,7 @@ def share_download(
     # Uniform "no user" for a missing OR non-messageable (demo/suspended/deactivated) target, so
     # sharing can't be used to probe which addresses exist or to reach a suspended account.
     if not is_messageable_user(recipient):
-        raise HTTPException(status_code=404, detail="No user with that email.")
+        raise not_found("recipient")
     if recipient.id == user.id:
         raise HTTPException(status_code=400, detail="That's already your download.")
     exists = db.execute(

backend/app/routes/scheduler.py

@@ -23,6 +23,7 @@ from app.scheduler import (
     trigger_job,
 )
 from app.sync.runner import estimate_deep_backfill
+from app.routes._common import not_found
 
 router = APIRouter(prefix="/api/admin/scheduler", tags=["admin"])
 
@@ -41,7 +42,7 @@ def set_job_interval(
     """Change how often a scheduler job runs (minutes). Persisted as an override and applied
     to the live scheduler immediately."""
     if job_id not in JOB_INTERVALS:
-        raise HTTPException(status_code=404, detail="Unknown job")
+        raise not_found("job")
     minutes = body.interval_minutes
     if not (MIN_INTERVAL <= minutes <= MAX_INTERVAL):
         raise HTTPException(
@@ -73,7 +74,7 @@ def run_job_now(
     try:
         result = trigger_job(job_id, actor_id=user.id)
     except KeyError:
-        raise HTTPException(status_code=404, detail="Unknown job")
+        raise not_found("job")
     if result == "already_running":
         raise HTTPException(status_code=409, detail="Job is already running")
     return {"id": job_id, "started": True}

backend/tests/test_not_found_detail.py

@@ -41,6 +41,39 @@ def test_a_gone_channel_video_or_user_says_what_is_gone(db, make_user, as_user):
         assert r.json()["detail"] == {"code": "not_found", "reason": what}, url
 
 
+def test_every_gone_channel_route_says_it_the_same_way(db, make_user, as_user):
+    """The round-4 sweep converted ONE of the five `db.get(Channel, …) is None -> 404 "Unknown
+    channel"` sites (block), and shipped the HU/EN string with it — so the Hungarian UI said it in
+    Hungarian on the Block button and in English on the four beside it (deep-review round 5). The
+    asymmetric fix is the smell this repo bans, so all five are pinned together."""
+    admin = make_user(role="admin")
+    client = as_user(admin)
+    gone = "UCgone000000000000000000"
+    # Subscribe is behind the write-scope gate, so it is covered by the tags route below instead.
+    for method, url in [
+        ("get", f"/api/channels/{gone}"),
+        ("post", f"/api/channels/{gone}/reset-backfill"),
+        ("post", f"/api/channels/{gone}/block"),
+    ]:
+        r = getattr(client, method)(url)
+        assert r.status_code == 404, (url, r.text)
+        assert r.json()["detail"] == {"code": "not_found", "reason": "channel"}, url
+
+
+def test_a_gone_subscription_recipient_or_job_says_what_is_gone(db, make_user, as_user):
+    """The three bare-English 404s the same sweep left behind, which `notifyActionError` printed
+    verbatim into an otherwise Hungarian UI: "Not subscribed to this channel", "No user with that
+    email.", "Unknown job"."""
+    admin = make_user(role="admin")
+    client = as_user(admin)
+    r = client.post("/api/channels/UCgone000000000000000000/tags", json={"tag_id": 1})
+    assert r.status_code == 404, r.text
+    assert r.json()["detail"] == {"code": "not_found", "reason": "subscription"}
+    r = client.patch("/api/admin/scheduler/jobs/no_such_job", json={"interval_minutes": 30})
+    assert r.status_code == 404, r.text
+    assert r.json()["detail"] == {"code": "not_found", "reason": "job"}
+
+
 def test_a_nul_in_a_request_string_is_a_422_not_a_500(db, make_user, as_user):
     """Postgres cannot hold NUL in text, so the driver refuses the query outright: a DataError, i.e.
     a 500 with a traceback, wherever a `%00` or an escaped NUL reached a comparison (deep-review

frontend/src/components/PlexCollectionEditor.tsx

@@ -77,6 +77,11 @@ export default function PlexCollectionEditor({
         return next;
       });
       refresh();
+    } catch (e) {
+      // 403/404 are left to the caller by `req` (see reportFailure), so without this the tick
+      // simply never appeared and the user clicked again — a collection deleted on the Plex server
+      // or flipped read-only failed in silence (deep-review round 5).
+      notifyActionError(e, t("plex.collection.editFailed"));
     } finally {
       setBusy(null);
     }
@@ -88,6 +93,8 @@ export default function PlexCollectionEditor({
     try {
       await api.plexSetCollectionEditable(id, true);
       refresh();
+    } catch (e) {
+      notifyActionError(e, t("plex.collection.editFailed"));
     } finally {
       setBusy(null);
     }
@@ -123,6 +130,10 @@ export default function PlexCollectionEditor({
         return next;
       });
       refresh();
+    } catch (e) {
+      // The worst of the three to lose: the user has just confirmed a destructive dialog, and the
+      // row stayed where it was with nothing said.
+      notifyActionError(e, t("plex.collection.deleteFailed"));
     } finally {
       setBusy(null);
     }

frontend/src/components/UploadDialog.tsx

@@ -5,6 +5,8 @@ import { UploadCloud, File as FileIcon } from "lucide-react";
 import Modal from "./ui/Modal";
 import { Button } from "./ui/Button";
 import { api } from "../lib/api";
+import { isAborted } from "../lib/api/core";
+import { notifyActionError } from "../lib/actionErrors";
 import { invalidateDownloads } from "../lib/downloads";
 import { notify } from "../lib/notifications";
 import { navigateTo } from "../lib/nav";
@@ -54,9 +56,15 @@ export default function UploadDialog({ onClose }: { onClose: () => void }) {
         action: { label: t("downloads.dialog.view"), onClick: () => navigateTo("downloads") },
       });
       onClose();
-    } catch {
-      // An aborted upload (Cancel) or a server rejection (cap/quota — already surfaced by the global
-      // error modal, like every other req call). Re-arm the form so the user can retry or pick again.
+    } catch (e) {
+      // Everything except the user's own Cancel is said out loud. `uploadReq` shares `reportFailure`
+      // with `req`, which deliberately leaves 403/404/429 to the caller and answers a 502/503/504
+      // with only the transient "connection lost" notice — so this empty catch meant a proxy 502 on
+      // a long upload, or an nginx 413, snapped the dialog back to its pre-upload state with nothing
+      // named and no job in the list. The comment here claimed the global error modal had it; it
+      // had not since the shared `reportFailure` landed (deep-review round 5).
+      if (!isAborted(e)) notifyActionError(e, t("downloads.upload.failed"));
+      // Re-arm the form so the user can retry or pick again.
       setPct(null);
       abortRef.current = null;
     }

frontend/src/components/WriteFailureReporting.dom.test.tsx

@@ -0,0 +1,146 @@
+// @vitest-environment jsdom
+//
+// A write that fails is named, wherever it fails. `req`/`uploadReq` deliberately leave 403/404/429
+// to the caller (see reportFailure) and answer a 502/503/504 with only the transient "connection
+// lost" notice — so a handler with no `catch` reports nothing at all. Round 4 swept a dozen such
+// call sites and missed these two: the collection editor's three siblings, in the very file it
+// edited, and the one `uploadReq` caller, whose empty catch still claimed the global error modal had
+// it (deep-review round 5).
+import { describe, it, expect, vi, beforeAll, beforeEach, afterEach } from "vitest";
+import { render, cleanup, screen, fireEvent, waitFor } from "@testing-library/react";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+
+vi.mock("react-i18next", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("react-i18next")>();
+  return {
+    ...actual,
+    useTranslation: () => ({ t: (key: string) => key, i18n: { language: "en" } }),
+  };
+});
+vi.mock("./ConfirmProvider", () => ({ useConfirm: () => async () => true }));
+const { notify } = vi.hoisted(() => ({
+  notify: vi.fn((_n: { level: string; message: string }) => 0),
+}));
+vi.mock("../lib/notifications", async (importOriginal) => ({
+  ...(await importOriginal<typeof import("../lib/notifications")>()),
+  notify,
+}));
+
+import PlexCollectionEditor from "./PlexCollectionEditor";
+import UploadDialog from "./UploadDialog";
+import { api, HttpError } from "../lib/api";
+
+beforeAll(() => {
+  class Stub {
+    observe() {}
+    unobserve() {}
+    disconnect() {}
+    takeRecords() {
+      return [];
+    }
+  }
+  globalThis.ResizeObserver ??= Stub as unknown as typeof ResizeObserver;
+  globalThis.IntersectionObserver ??= Stub as unknown as typeof IntersectionObserver;
+});
+beforeEach(() => notify.mockClear());
+afterEach(() => {
+  cleanup();
+  vi.restoreAllMocks();
+});
+
+const errors = () => notify.mock.calls.filter(([n]) => n.level === "error").map(([n]) => n.message);
+const client = () => new QueryClient({ defaultOptions: { queries: { retry: false } } });
+
+describe("the Plex collection editor", () => {
+  const collections = [
+    { id: "c1", title: "Marathon", can_edit: true, child_count: 3, smart: false },
+    // Not marked "mine to edit" yet, so it offers the take-over button.
+    {
+      id: "c2",
+      title: "Theirs",
+      can_edit: false,
+      child_count: 9,
+      smart: false,
+      source: "collection",
+    },
+  ];
+  const mount = () =>
+    render(
+      <QueryClientProvider client={client()}>
+        <PlexCollectionEditor
+          item={{ id: "m1", title: "A Movie" }}
+          library="1"
+          memberOf={[]}
+          onClose={() => {}}
+          onChanged={() => {}}
+        />
+      </QueryClientProvider>,
+    );
+
+  it("says so when adding to a collection that is gone, instead of nothing happening", async () => {
+    vi.spyOn(api, "plexCollections").mockResolvedValue({ collections } as never);
+    vi.spyOn(api, "plexCollectionAddItem").mockRejectedValue(
+      new HttpError(404, "Unknown collection"),
+    );
+    mount();
+    fireEvent.click(await screen.findByText("Marathon"));
+    await waitFor(() => expect(errors()).toEqual(["plex.collection.editFailed"]));
+  });
+
+  it("says so when enabling editing is refused", async () => {
+    vi.spyOn(api, "plexCollections").mockResolvedValue({ collections } as never);
+    vi.spyOn(api, "plexSetCollectionEditable").mockRejectedValue(
+      new HttpError(403, "This collection is read-only"),
+    );
+    mount();
+    fireEvent.click(await screen.findByText("plex.collEditor.makeEditable"));
+    await waitFor(() => expect(errors()).toEqual(["plex.collection.editFailed"]));
+  });
+
+  it("says so when a CONFIRMED delete does not happen", async () => {
+    // The worst of the three to lose: the user has just confirmed a destructive dialog.
+    vi.spyOn(api, "plexCollections").mockResolvedValue({ collections } as never);
+    vi.spyOn(api, "plexDeleteCollection").mockRejectedValue(
+      new HttpError(404, "Unknown collection"),
+    );
+    mount();
+    fireEvent.click(await screen.findByText("plex.collEditor.delete"));
+    await waitFor(() => expect(errors()).toEqual(["plex.collection.deleteFailed"]));
+  });
+});
+
+describe("the upload dialog", () => {
+  const pick = () => {
+    const input = document.querySelector('input[type="file"]') as HTMLInputElement;
+    const file = new File(["x"], "clip.mp4", { type: "video/mp4" });
+    Object.defineProperty(input, "files", { value: [file], configurable: true });
+    fireEvent.change(input);
+  };
+  const mount = () =>
+    render(
+      <QueryClientProvider client={client()}>
+        <UploadDialog onClose={() => {}} />
+      </QueryClientProvider>,
+    );
+
+  it("names an upload the server refused", async () => {
+    // A 502 from the proxy raises only the self-clearing connection notice; the next successful poll
+    // wipes it, and the dialog snapped back with no error and no job in the list.
+    vi.spyOn(api, "uploadDownload").mockRejectedValue(new HttpError(502, "Bad Gateway"));
+    mount();
+    pick();
+    fireEvent.click(screen.getByText("downloads.upload.submit"));
+    await waitFor(() => expect(errors()).toEqual(["downloads.upload.failed"]));
+  });
+
+  it("says nothing when the user cancelled it themselves", async () => {
+    // A cancel is not a failure — and it rejects with the same status 0 a genuine network drop does,
+    // which is why the difference is a named sentinel (`isAborted`) and not a status check.
+    const upload = vi.spyOn(api, "uploadDownload").mockRejectedValue(new HttpError(0, "aborted"));
+    mount();
+    pick();
+    fireEvent.click(screen.getByText("downloads.upload.submit"));
+    await waitFor(() => expect(upload).toHaveBeenCalled());
+    expect(errors()).toEqual([]);
+  });
+});

frontend/src/components/usePlaylistActions.dom.test.tsx

@@ -60,10 +60,25 @@ describe("usePlaylistActions — deleting a list", () => {
     expect(h.notify).not.toHaveBeenCalled();
   });
 
-  it("does say one it was left to handle", async () => {
-    vi.spyOn(api, "deletePlaylist").mockRejectedValue(new HttpError(404, "gone"));
+  it("does say one it was left to handle — in the reader's language", async () => {
+    // A plain-string `detail` is whatever English the route hand-wrote; the caller's own translated
+    // fallback is the one a Hungarian reader can read (deep-review round 5).
+    vi.spyOn(api, "deletePlaylist").mockRejectedValue(new HttpError(404, "Unknown playlist"));
     const { result } = mount();
     await act(() => result.current.remove(linked));
-    expect(h.notify).toHaveBeenCalledWith(expect.objectContaining({ message: "gone" }));
+    expect(h.notify).toHaveBeenCalledWith(
+      expect.objectContaining({ message: "playlists.deleteYtFailed" }),
+    );
+  });
+
+  it("prefers a STRUCTURED reason, which the request layer has already localized", async () => {
+    vi.spyOn(api, "deletePlaylist").mockRejectedValue(
+      new HttpError(404, "Ez a lista már nem létezik.", { code: "not_found", reason: "playlist" }),
+    );
+    const { result } = mount();
+    await act(() => result.current.remove(linked));
+    expect(h.notify).toHaveBeenCalledWith(
+      expect.objectContaining({ message: "Ez a lista már nem létezik." }),
+    );
   });
 });

frontend/src/i18n/locales/en/downloads.json

@@ -34,7 +34,8 @@
     "submit": "Upload",
     "uploading": "Uploading… {{pct}}%",
     "uploadProgress": "Upload progress",
-    "added": "File uploaded"
+    "added": "File uploaded",
+    "failed": "The upload failed."
   },
   "tabs": {
     "queue": "Queue",

frontend/src/i18n/locales/en/errors.json

@@ -14,7 +14,10 @@
     "video": "This video isn't in the library any more.",
     "invite": "This invite doesn't exist any more.",
     "user": "This user doesn't exist any more.",
-    "channel": "This channel isn't in the library any more."
+    "channel": "This channel isn't in the library any more.",
+    "subscription": "You're not subscribed to this channel any more.",
+    "recipient": "There's no user with that email address.",
+    "job": "This job doesn't exist any more."
   },
   "ok": "OK",
   "offline": {

frontend/src/i18n/locales/en/plex.json

@@ -29,7 +29,8 @@
     "namePlaceholder": "Collection name",
     "nameTaken": "A collection with this name already exists.",
     "newHint": "Pick the first title to add — a collection needs at least one.",
-    "createWith": "Create"
+    "createWith": "Create",
+    "deleteFailed": "Couldn't delete the collection."
   },
   "searchPlaceholder": "Search Plex…",
   "searchField": {

frontend/src/i18n/locales/hu/downloads.json

@@ -34,7 +34,8 @@
     "submit": "Feltöltés",
     "uploading": "Feltöltés… {{pct}}%",
     "uploadProgress": "Feltöltés folyamata",
-    "added": "Fájl feltöltve"
+    "added": "Fájl feltöltve",
+    "failed": "A feltöltés nem sikerült."
   },
   "tabs": {
     "queue": "Sor",

frontend/src/i18n/locales/hu/errors.json

@@ -14,7 +14,10 @@
     "video": "Ez a videó már nincs a könyvtárban.",
     "invite": "Ez a meghívó már nem létezik.",
     "user": "Ez a felhasználó már nem létezik.",
-    "channel": "Ez a csatorna már nincs a könyvtárban."
+    "channel": "Ez a csatorna már nincs a könyvtárban.",
+    "subscription": "Már nem vagy feliratkozva erre a csatornára.",
+    "recipient": "Nincs felhasználó ezzel az e-mail címmel.",
+    "job": "Ez a feladat már nem létezik."
   },
   "ok": "OK",
   "offline": {

frontend/src/i18n/locales/hu/plex.json

@@ -29,7 +29,8 @@
     "namePlaceholder": "Kollekció neve",
     "nameTaken": "Már létezik ilyen nevű kollekció.",
     "newHint": "Válaszd ki az első címet — egy kollekcióhoz legalább egy elem kell.",
-    "createWith": "Létrehozás"
+    "createWith": "Létrehozás",
+    "deleteFailed": "Nem sikerült törölni a gyűjteményt."
   },
   "searchPlaceholder": "Keresés a Plexben…",
   "searchField": {

frontend/src/lib/actionErrors.test.ts

@@ -57,6 +57,28 @@ describe("notifyActionError", () => {
 
   it("does toast one the caller was left to handle", () => {
     notifyActionError(new HttpError(404, "gone"), "FAILED");
-    expect(notifyMock).toHaveBeenCalledWith({ level: "error", message: "gone" });
+    expect(notifyMock).toHaveBeenCalled();
+  });
+
+  it("says a bare-English server reason in the reader's language, not the server's", () => {
+    // `reportFailure` leaves 403/404/429 to the caller, so for exactly those statuses whatever
+    // English a route hand-wrote went straight into the toast — "No user with that email." in an
+    // otherwise Hungarian UI, while the caller's translated fallback sat unused (round 5).
+    notifyActionError(new HttpError(404, "No user with that email."), "TRANSLATED");
+    expect(notifyMock).toHaveBeenCalledWith({ level: "error", message: "TRANSLATED" });
+  });
+
+  it("still prefers a STRUCTURED reason, which the request layer has already localized", () => {
+    // `req` runs `{code, reason}` through localizeDetail, so `detail` is in the reader's language
+    // AND more specific than the caller's generic fallback — quota numbers, which row is gone.
+    const err = new HttpError(404, "Ez a csatorna már nincs a könyvtárban.", {
+      code: "not_found",
+      reason: "channel",
+    });
+    notifyActionError(err, "TRANSLATED FALLBACK");
+    expect(notifyMock).toHaveBeenCalledWith({
+      level: "error",
+      message: "Ez a csatorna már nincs a könyvtárban.",
+    });
   });
 });

frontend/src/lib/actionErrors.ts

@@ -5,10 +5,14 @@ import { notify } from "./notifications";
 
 /**
  * Surface a failed write action as an error toast instead of the click silently doing nothing.
- * Prefers the server's own reason (`HttpError.detail` — e.g. "Not enough quota left today", a
- * 404/429 message) over the caller's already-translated generic fallback. Domain-agnostic: used by
- * the Plex watched-toggles and the saved-view ops, and delegated to by `notifyYouTubeActionError`
- * for its non-403 case.
+ * Prefers the server's own reason over the caller's already-translated generic fallback — but only
+ * when the server sent a STRUCTURED detail (`{code, reason}`), which `req` has already run through
+ * `localizeDetail`. A plain-string `detail` is whatever English the route hand-wrote, and
+ * `reportFailure` deliberately leaves 403/404/429 to the caller, so exactly those statuses used to
+ * put the route's English sentence into an otherwise Hungarian UI — "No user with that email.",
+ * "Not subscribed to this channel" — where the caller's own translated fallback was right there
+ * (deep-review round 5). Domain-agnostic: used by the Plex watched-toggles and the saved-view ops,
+ * and delegated to by `notifyYouTubeActionError` for its non-403 case.
  */
 export function notifyActionError(
   err: unknown,
@@ -19,6 +23,6 @@ export function notifyActionError(
   // top reported one fault twice. Playlists filtered this by status by hand; the watched toggles,
   // the saved views and a playlist delete did not.
   if (alreadyReported(err)) return;
-  const detail = err instanceof HttpError ? err.detail : null;
-  notify({ level, message: detail || fallbackMessage });
+  const localized = err instanceof HttpError && err.detailData != null ? err.detail : null;
+  notify({ level, message: localized || fallbackMessage });
 }

frontend/src/lib/api/core.ts

@@ -28,6 +28,16 @@ export function alreadyReported(err: unknown): boolean {
   return typeof err === "object" && err !== null && reported.has(err);
 }
 
+/** What an upload the USER cancelled rejects with. A cancel is not a failure to report, and the
+ *  only other rejection carrying status 0 is a genuine network drop, which IS worth reporting — so
+ *  the difference has to be a named thing both sides agree on, not a string literal in each. */
+const ABORTED = "aborted";
+
+/** True for the rejection of a request the user cancelled (see `ABORTED`). */
+export function isAborted(err: unknown): boolean {
+  return err instanceof HttpError && err.status === 0 && err.detail === ABORTED;
+}
+
 export class HttpError extends Error {
   status: number;
   detail?: string; // server-provided reason (FastAPI's `detail`), when present
@@ -342,7 +352,7 @@ export function uploadReq<T = unknown>(
       };
     }
     if (cfg.signal) {
-      if (cfg.signal.aborted) return reject(new HttpError(0, "aborted"));
+      if (cfg.signal.aborted) return reject(new HttpError(0, ABORTED));
       cfg.signal.addEventListener("abort", () => xhr.abort());
     }
     xhr.onload = () => {
@@ -376,7 +386,7 @@ export function uploadReq<T = unknown>(
       markConnectivityLost();
       reject(new HttpError(0, i18n.t("errors.offline.body")));
     };
-    xhr.onabort = () => reject(new HttpError(0, "aborted"));
+    xhr.onabort = () => reject(new HttpError(0, ABORTED));
     xhr.send(form);
   });
 }