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/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);
   });
 }