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/components/Playlists.tsx

@@ -32,7 +32,7 @@ import {
   Upload,
   Youtube,
 } from "lucide-react";
-import { api, type Video } from "../lib/api";
+import { api, HttpError, type Video } from "../lib/api";
 import { notify } from "../lib/notifications";
 import { notifyYouTubeActionError } from "../lib/youtubeErrors";
 import { playlistName } from "../lib/playlistName";
@@ -489,10 +489,14 @@ export default function Playlists() {
     setSelectedId(remaining[0]?.id ?? null);
     try {
       await api.deletePlaylist(deletedId, onYoutube);
-    } catch {
+    } catch (err) {
+      // Prefer the server's specific reason (e.g. a local vs YouTube-side failure) over a blanket
+      // "couldn't delete on YouTube" that misleads when the failure wasn't the YouTube side.
       notify({
         level: "warning",
-        message: onYoutube ? t("playlists.deleteYtFailed") : t("playlists.deleteFailed"),
+        message:
+          (err instanceof HttpError && err.detail) ||
+          (onYoutube ? t("playlists.deleteYtFailed") : t("playlists.deleteFailed")),
       });
     }
     qc.removeQueries({ queryKey: ["playlist", deletedId] });