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/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}