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/lib/youtubeErrors.ts

@@ -11,7 +11,9 @@ export function notifyYouTubeActionError(
   fallbackMessage: string,
   onConnect?: () => void,
 ): void {
-  if (err instanceof HttpError && err.status === 403) {
+  const httpErr = err instanceof HttpError ? err : null;
+  if (httpErr?.status === 403) {
+    // A 403 on a YouTube write action means the write scope isn't granted — offer to connect.
     notify({
       level: "error",
       message: i18n.t("channels.notify.needYouTube"),
@@ -19,7 +21,9 @@ export function notifyYouTubeActionError(
         ? { label: i18n.t("channels.notify.connect"), onClick: onConnect }
         : undefined,
     });
-  } else {
-    notify({ level: "error", message: fallbackMessage });
+    return;
   }
+  // Any other error: prefer the server's own reason (e.g. "Not enough quota left today") over the
+  // caller's generic fallback — the useful detail was being discarded (U-3.2.8).
+  notify({ level: "error", message: httpErr?.detail || fallbackMessage });
 }