feat(web/player): self-heal a stale system-playlist / radio queue on total failure

When the whole queue proves unplayable (e.g. a tab left open across the
daily system-playlist rebuild — the exact stale-snapshot case), the player
now re-pulls the fresh snapshot and resumes instead of dead-ending on
"Try again". The seeder hands the store an opaque refetch closure so the
store stays decoupled from the playlist API and the per-artist
(songs_like_artist) identity problem: single-instance variants re-pull via
systemShuffle, per-artist mixes via getPlaylist(id), radio re-seeds from
its track. Bounded to one self-heal per exhaustion (reset on the next
successful play) so a still-broken refresh can't loop; "Try again" stays
the genuine last resort. Wired from PlaylistCard, the playlist detail page,
and playRadio. Issue #968.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/lib/playlists/systemRefetch.ts

@@ -0,0 +1,33 @@
+import type { TrackRef, PlaylistDetail } from '$lib/api/types';
+import { systemShuffle, getPlaylist } from '$lib/api/playlists';
+import { playlistTrackToRef } from './playlistTrackToRef';
+
+// #968: builds the self-heal closure for a system-playlist queue. On total
+// playback failure (every queued track unplayable — typically a stale tab
+// left open across the daily rebuild) the player calls this to re-pull the
+// current snapshot and resume.
+//
+// Single-instance variants (for_you, discover, deep_cuts, …) re-pull by
+// variant via the rotation-aware shuffle endpoint. Per-artist mixes
+// (songs_like_artist — one playlist per seed artist) can't be addressed by
+// variant alone, so they re-pull by playlist id. Mirrors the play routing
+// in PlaylistCard.
+
+function toRefs(detail: PlaylistDetail): TrackRef[] {
+  return detail.tracks
+    .map((r) => playlistTrackToRef(r))
+    .filter((t): t is TrackRef => t !== null);
+}
+
+export function systemPlaylistRefetch(opts: {
+  variant: string;
+  playlistId: string;
+  perArtist: boolean;
+}): () => Promise<TrackRef[]> {
+  return async () => {
+    const detail = opts.perArtist
+      ? await getPlaylist(opts.playlistId)
+      : await systemShuffle(opts.variant);
+    return toRefs(detail);
+  };
+}