fix(errors,a11y): review round 1 — drop redundant onErrors, stabilize the live region

Review found my C-3.23 additions double-notified: the global error modal (api.ts req)
already surfaces 400/409/422/500 with the backend's specific message, so a caller
toast on those codes is a second, weaker surface. Only 403/404 are caller-handled and
silent — and only a YouTube WRITE action has a meaningful 403 (missing scope).

So the correct set is just the ChannelPage unsubscribe handler (a YouTube write, 403 →
"connect", matching subscribe). Reverted the redundant onErrors on block (a local DB
op, not YouTube), SettingsPanel Plex watch toggle/reimport, and the DownloadCenter job
action — the global modal already speaks for them. Removed the three now-unused keys.

Toaster: moved the polite live region onto the stable, always-mounted container (a
polite region must pre-exist to announce later insertions); error/fatal toasts keep
role="alert", which announces on insertion regardless.

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

@@ -119,7 +119,6 @@
     "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

@@ -197,6 +197,5 @@
       "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

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