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/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": {