feat(ux): R11 feedback symmetry — Plex/download/rollback/deny notices

Give equal-weight actions equal-weight feedback (R11).

S1 Plex error toasts: the watched-toggles (card / season mark-all / episode /
info-panel / show mark-all) used to fail silently on 403/404/429; each now surfaces
the server reason via a shared notifyActionError() + resyncs the optimistic flip.
The playback auto-mark-watched stays intentionally silent (a mid-play toast is
noise; the next sync corrects it). New generic lib/actionErrors.ts (Plex + saved
views reuse it; notifyYouTubeActionError delegates its non-403 case to it).

S2 download-done inbox notification: worker.py emits a best-effort type=download_done
notice to the job owner on real completion (regular + edit jobs, not instant
cache-hits), rendered/localized in the inbox with an Open-Downloads link.

S3 optimistic rollbacks: SavedViewsWidget create/rename/toggle-default/remove/reorder
and the inbox revert-state no longer swallow failures — they toast + refetch to
resync (the reorder's optimistic order reverts to server truth).

S4 access-request deny: deny_invite now emails the requester (send_access_denied),
mirroring approve — no more silent limbo.

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

@@ -12,6 +12,11 @@
   "sectionActivity": "Activity",
   "andMore": "and {{count}} more",
   "openScheduler": "Open Scheduler",
+  "openDownloads": "Open Downloads",
+  "revertFailed": "Couldn't undo that — please try again.",
+  "downloadDone": {
+    "title": "Download ready"
+  },
   "maintenance": {
     "title": "Videos removed",
     "body_one": "{{count}} saved or playlisted video was removed because it's no longer available on YouTube.",

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

@@ -74,6 +74,7 @@
   "openShow": "Open show",
   "markWatched": "Mark watched",
   "markUnwatched": "Mark unwatched",
+  "watchedFailed": "Couldn't update the watched state — please try again.",
   "seasons": "{{count}} seasons",
   "unified": {
     "titles": "Titles",

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

@@ -12,5 +12,7 @@
   "rename": "Rename",
   "delete": "Delete",
   "deleteConfirm": "Delete the saved view “{{name}}”?",
-  "reorder": "Drag to reorder"
+  "reorder": "Drag to reorder",
+  "actionFailed": "Couldn't update your saved views — please try again.",
+  "reorderFailed": "Couldn't save the new order — please try again."
 }

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

@@ -12,6 +12,11 @@
   "sectionActivity": "Tevékenység",
   "andMore": "és még {{count}}",
   "openScheduler": "Ütemező megnyitása",
+  "openDownloads": "Letöltések megnyitása",
+  "revertFailed": "Nem sikerült visszavonni — próbáld újra.",
+  "downloadDone": {
+    "title": "Letöltés kész"
+  },
   "maintenance": {
     "title": "Videók eltávolítva",
     "body_one": "{{count}} mentett vagy lejátszási listás videó eltávolítva, mert már nem elérhető a YouTube-on.",

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

@@ -74,6 +74,7 @@
   "openShow": "Sorozat megnyitása",
   "markWatched": "Megnézettnek jelöl",
   "markUnwatched": "Nem-nézettnek jelöl",
+  "watchedFailed": "Nem sikerült frissíteni a megnézett állapotot — próbáld újra.",
   "seasons": "{{count}} évad",
   "unified": {
     "titles": "Címek",

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

@@ -12,5 +12,7 @@
   "rename": "Átnevezés",
   "delete": "Törlés",
   "deleteConfirm": "Törlöd a(z) „{{name}}” mentett nézetet?",
-  "reorder": "Húzd az átrendezéshez"
+  "reorder": "Húzd az átrendezéshez",
+  "actionFailed": "Nem sikerült frissíteni a mentett nézeteket — próbáld újra.",
+  "reorderFailed": "Nem sikerült menteni az új sorrendet — próbáld újra."
 }