feat(flutter): playlist cover art via deterministic /api/playlists/{id}/cover

Same fix shape as albums: drift's CachedPlaylistAdapter.toRef was
returning coverUrl: '' because the cache table doesn't persist the
server-derived URL. Set it to /api/playlists/<id>/cover in the
adapter — handleGetPlaylistCover serves the cached collage from
disk, so the URL is deterministic and the round-trip through drift
no longer drops it.

PlaylistCard + PlaylistsListScreen pass the existing queue_music
icon as ServerImage's fallback, so when the server hasn't built a
collage yet (system playlists with no tracks at build time), the
endpoint 404s and the icon shows over the slate background instead
of an empty box.

flutter_client/lib/playlists/widgets/playlist_card.dart

@@ -31,9 +31,19 @@ class PlaylistCard extends StatelessWidget {
                   width: 144,
                   height: 144,
                   color: fs.slate,
+                  // coverUrl is deterministic per playlist (see
+                  // CachedPlaylistAdapter.toRef). When the server's
+                  // collage isn't built yet, ServerImage's
+                  // errorBuilder shows the queue_music icon over the
+                  // slate background.
                   child: playlist.coverUrl.isEmpty
                       ? Icon(Icons.queue_music, color: fs.ash, size: 56)
-                      : ServerImage(url: playlist.coverUrl, fit: BoxFit.cover),
+                      : ServerImage(
+                          url: playlist.coverUrl,
+                          fit: BoxFit.cover,
+                          fallback:
+                              Icon(Icons.queue_music, color: fs.ash, size: 56),
+                        ),
                 ),
               ),
               const SizedBox(height: 8),