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.

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;
     }