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/routes/playlists/[id]/+page.svelte

@@ -18,6 +18,7 @@
   import { user } from '$lib/auth/store.svelte';
   import { errCode, errMessage } from '$lib/api/errors';
   import { playQueue } from '$lib/player/store.svelte';
+  import { systemPlaylistRefetch } from '$lib/playlists/systemRefetch';
   import { pushToast } from '$lib/stores/toast.svelte';
   import type { TrackRef } from '$lib/api/types';
 
@@ -61,16 +62,30 @@
 
   function onPlay(position: number) {
     if (!playlistQuery?.data) return;
+    const data = playlistQuery.data;
     // Skip rows where track_id is null (track was removed from library).
-    const live = playlistQuery.data.tracks
-      .filter((t) => t.track_id !== null)
-      .map(toTrackRef);
+    const live = data.tracks.filter((t) => t.track_id !== null).map(toTrackRef);
     // Find which `live` index corresponds to the clicked position. Some
     // positions in the original list may be unavailable (filtered out),
     // so the index in the playable list != the playlist position.
-    const clickedTrackID = playlistQuery.data.tracks.find((t) => t.position === position)?.track_id;
+    const clickedTrackID = data.tracks.find((t) => t.position === position)?.track_id;
     const startIdx = live.findIndex((t) => t.id === clickedTrackID);
-    playQueue(live, Math.max(0, startIdx));
+    // #968: a system playlist played from its detail page gets the same
+    // self-heal closure as the home tile, so a stale snapshot re-pulls on
+    // total failure. (Source attribution is intentionally left to the home
+    // tile / shuffle path — this only adds the recovery closure.)
+    const variant = data.system_variant;
+    if (variant != null) {
+      playQueue(live, Math.max(0, startIdx), {
+        refetch: systemPlaylistRefetch({
+          variant,
+          playlistId: id,
+          perArtist: data.seed_artist_id != null
+        })
+      });
+    } else {
+      playQueue(live, Math.max(0, startIdx));
+    }
   }
 
   function toTrackRef(t: {