fix(errors): surface four mutations that failed silently (C-3.23)

House policy: user actions must report failure. Added onError to:
- ChannelPage block + unsubscribe → notifyYouTubeActionError (a 403 = missing YT
  scope shows the connect message; else the translated fallback);
- SettingsPanel PlexWatchSync toggle + reimport → a plain error notify (these are
  Plex, not YouTube, so notifyYouTubeActionError's "connect YouTube" would mislead);
- DownloadCenter job action (pause/resume/cancel/delete) → a plain error notify.
New keys (HU+EN): channels.notify.blockFailed, settings.plexSync.failed,
downloads.actionFailed.

frontend/src/components/ChannelPage.tsx

@@ -120,6 +120,7 @@ export default function ChannelPage({
       qc.invalidateQueries({ queryKey: ["feed"] });
       qc.invalidateQueries({ queryKey: ["blocked-channels"] });
     },
+    onError: (err) => notifyYouTubeActionError(err, t("channels.notify.blockFailed")),
   });
 
   const subscribe = useMutation({
@@ -144,6 +145,7 @@ export default function ChannelPage({
       qc.invalidateQueries({ queryKey: ["feed"] });
       qc.invalidateQueries({ queryKey: ["channels"] });
     },
+    onError: (err) => notifyYouTubeActionError(err, t("channels.notify.unsubscribeFailed")),
   });
   const onUnsubscribe = async () => {
     const ok = await confirm({

frontend/src/components/DownloadCenter.tsx

@@ -681,6 +681,7 @@ export default function DownloadCenter() {
             ? api.cancelDownload(id)
             : api.deleteDownload(id),
     onSuccess: invalidate,
+    onError: () => notify({ level: "error", message: t("downloads.actionFailed") }),
   });
 
   // Remove a shared-with-me item from your list (only your grant; the owner's file is untouched).

frontend/src/components/SettingsPanel.tsx

@@ -533,12 +533,14 @@ function PlexWatchSync() {
     }
   };
 
+  const onFail = () => notify({ level: "error", message: t("settings.plexSync.failed") });
   const toggle = useMutation({
     mutationFn: (next: boolean) => api.plexWatchSetLink(next),
     onSuccess: (res) => {
       qc.setQueryData(["plex-watch-link"], res);
       announce(res.import);
     },
+    onError: onFail,
   });
   const reimport = useMutation({
     mutationFn: () => api.plexWatchImport(),
@@ -546,6 +548,7 @@ function PlexWatchSync() {
       qc.setQueryData(["plex-watch-link"], res);
       announce(res.import);
     },
+    onError: onFail,
   });
 
   const busy = toggle.isPending || reimport.isPending;

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

@@ -119,6 +119,7 @@
     "syncFailed": "Subscription sync failed",
     "unsubscribed": "Unsubscribed from {{name}} on YouTube",
     "unsubscribeFailed": "Unsubscribe failed",
+    "blockFailed": "Couldn't update block state",
     "fullHistoryRequested": "Full history requested for {{count}} channels",
     "fullHistoryFailed": "Couldn't request full history",
     "needYouTube": "Connect your YouTube account to do this.",

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

@@ -197,5 +197,6 @@
       "source_not_ready": "The source download isn't ready to edit.",
       "generic": "That edit couldn't be created."
     }
-  }
+  },
+  "actionFailed": "Download action failed"
 }

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

@@ -14,7 +14,8 @@
     "imported": "Imported from Plex: {{watched}} watched, {{in_progress}} in progress.",
     "lastSync": "Last import: {{when}}",
     "never": "Not imported yet",
-    "importing": "Importing your Plex watch history…"
+    "importing": "Importing your Plex watch history…",
+    "failed": "Plex watch sync failed"
   },
   "save": {
     "saving": "Saving…",

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

@@ -119,6 +119,7 @@
     "syncFailed": "A feliratkozások szinkronizálása sikertelen",
     "unsubscribed": "Leiratkozva a(z) {{name}} csatornáról a YouTube-on",
     "unsubscribeFailed": "A leiratkozás sikertelen",
+    "blockFailed": "A tiltás állapotát nem sikerült frissíteni",
     "fullHistoryRequested": "Teljes előzmény kérve {{count}} csatornához",
     "fullHistoryFailed": "Nem sikerült teljes előzményt kérni",
     "needYouTube": "Ehhez csatlakoztasd a YouTube-fiókod.",

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

@@ -197,5 +197,6 @@
       "source_not_ready": "A forrásletöltés még nem szerkeszthető.",
       "generic": "Ezt a vágást nem sikerült létrehozni."
     }
-  }
+  },
+  "actionFailed": "A letöltési művelet sikertelen"
 }

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

@@ -14,7 +14,8 @@
     "imported": "Importálva a Plexből: {{watched}} megnézve, {{in_progress}} folyamatban.",
     "lastSync": "Utolsó import: {{when}}",
     "never": "Még nincs importálva",
-    "importing": "Plex-előzmény importálása…"
+    "importing": "Plex-előzmény importálása…",
+    "failed": "A Plex-megtekintés szinkron sikertelen"
   },
   "save": {
     "saving": "Mentés…",