fix(player): seed full-player display state from current mediaItem on mount

Regression from v2026.05.13.2's load-then-swap rewrite. _displayedMedia
only got populated by the ref.listen callback on mediaItem changes,
but ref.listen doesn't fire on initial subscription — it only fires
on transitions after the listener is registered. So opening the full
player while a track was already playing left _displayedMedia null
and the screen rendered "Nothing playing." even though the mini bar
showed a live track.

initState now reads the current mediaItem synchronously and seeds
_displayedMedia immediately (and _displayedDominant from the color
provider's cached value when available). A post-frame
_scheduleSwap(current) runs to ensure the cover bytes are decoded
and dominant color resolved when the user opens the player to a
track whose album hasn't yet been color-extracted in this session.

flutter_client/lib/player/now_playing_screen.dart

@@ -62,6 +62,39 @@ class _NowPlayingScreenState extends ConsumerState<NowPlayingScreen> {
   /// a track whose cover hadn't finished decoding yet.
   String? _pendingPreloadId;
 
+  @override
+  void initState() {
+    super.initState();
+    // Seed displayed state from the current mediaItem if a track is
+    // already playing when the full player is opened. ref.listen
+    // below only fires on CHANGES after subscription, so without
+    // this seed the screen sits on "Nothing playing." even though
+    // the mini bar shows a live track — the listener never gets a
+    // mediaItem emission because the stream's current value hasn't
+    // changed.
+    final current = ref.read(mediaItemProvider).value;
+    if (current == null) return;
+    _displayedMedia = current;
+    // Best-effort synchronous color seed: if albumColorProvider has
+    // already extracted this album's dominant color earlier in the
+    // session, pull it now so the backdrop renders correctly on
+    // first frame. Otherwise the post-frame _scheduleSwap below
+    // resolves it asynchronously and AnimatedContainer tweens.
+    final albumId = current.extras?['album_id'] as String?;
+    if (albumId != null && albumId.isNotEmpty) {
+      _displayedDominant =
+          ref.read(albumColorProvider(albumId)).asData?.value;
+    }
+    // Kick the preload pipeline post-frame so the cover bytes are
+    // decoded + the dominant color is resolved even when the user
+    // opens the player to a track that hasn't yet flowed through
+    // ref.listen.
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      if (!mounted) return;
+      _scheduleSwap(current);
+    });
+  }
+
   /// Preload the new track's cover bytes + dominant color, then
   /// atomically flip _displayedMedia / _displayedDominant. Until both
   /// resolve, the screen continues to render the previous track —