feat(ui): R3 S3 — surface swallowed errors

- notifyYouTubeActionError: on any non-403 error, show the server's own reason (err.detail, e.g.
  "Not enough quota left today") instead of the caller's generic fallback, which was discarded
  (U-3.2.8). 403 still offers the Connect affordance.
- App boot error now offers a Retry (StateMessage) instead of a dead "Something went wrong" (U-4.2).
- deletePlaylist prefers the server's reason over a blanket "couldn't delete on YouTube" that
  misleads when the failure wasn't the YouTube side.
- Already covered in earlier sprints (verified): the live-search "Load more" already surfaces the
  quota error inline via err.detail, and ChannelPage subscribe/unsubscribe already route through
  notifyYouTubeActionError.

frontend/src/App.tsx

@@ -31,6 +31,7 @@ import PageShell from "./components/PageShell";
 import GlassTuner from "./components/GlassTuner";
 import ChatDock from "./components/ChatDock";
 import Toaster from "./components/Toaster";
+import { StateMessage } from "./components/QueryState";
 import ErrorDialog from "./components/ErrorDialog";
 import VersionBanner from "./components/VersionBanner";
 import DemoBanner from "./components/DemoBanner";
@@ -294,8 +295,10 @@ export default function App() {
   // network/5xx failure, and no data means "not signed in" → the public landing.
   if (meQuery.error)
     return (
-      <div className="min-h-screen grid place-items-center text-muted">
-        {t("common.somethingWrong")}
+      <div className="min-h-screen grid place-items-center">
+        <StateMessage tone="error" onRetry={() => meQuery.refetch()}>
+          {t("common.somethingWrong")}
+        </StateMessage>
       </div>
     );
   if (!meQuery.data) return <Welcome />;