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/components/PlexInfo.tsx

@@ -5,6 +5,7 @@ import { useQueryClient } from "@tanstack/react-query";
 import { Check, ExternalLink, FolderPlus, ListPlus, Play, RotateCcw, Star, X } from "lucide-react";
 import { api, type PlexFilters, type PlexItemDetail } from "../lib/api";
 import { formatRuntime } from "../lib/format";
+import { notifyActionError } from "../lib/actionErrors";
 import {
   DetailCustomizeMenu,
   Filterable,
@@ -77,7 +78,11 @@ export default function PlexInfo({
 
   const inProgress = (detail.position_seconds ?? 0) > 0 && detail.status !== "watched";
   const setState = async (status: "new" | "watched") => {
-    await api.plexSetState(detail.id, status).catch(() => {});
+    try {
+      await api.plexSetState(detail.id, status);
+    } catch (e) {
+      notifyActionError(e, t("plex.watchedFailed"));
+    }
     qc.invalidateQueries({ queryKey: qk.plexItem(detail.id) });
     qc.invalidateQueries({ queryKey: qk.plexLibrary() });
     qc.invalidateQueries({ queryKey: qk.plexShow() });