fix(player): smooth mini bar cover swap across track change

Audio handler broadcasts MediaItem twice on every track change:
once with artUri=null (the new track's bare metadata), then again
with artUri pointing at the AlbumCoverCache file once the cover
lands on disk. The mini player's cover element was rebuilding in
place: previous track's image → slate placeholder → new track's
image. That flash is the flicker reported on the v2026.05.13.0 build.

AnimatedSwitcher around the cover (180ms crossfade) keyed by the
artUri value makes the swap a smooth crossfade instead of a visible
snap. The Hero parent stays — its tag is stable across track changes
so the mini→full bar expansion animation keeps working unchanged.

flutter_client/lib/player/player_bar.dart

@@ -95,14 +95,25 @@ class _TrackInfo extends StatelessWidget {
     // artwork from this 48dp footprint to the full-screen size rather
     // than fade-cutting. Tag is stable per-route (not keyed by media.id)
     // so the transition works regardless of what's playing.
-    Widget cover;
+    //
+    // AnimatedSwitcher around the cover smooths the artUri null→file://
+    // swap that fires on every track change: audio_handler broadcasts
+    // the new MediaItem without artUri immediately, then re-broadcasts
+    // with artUri once AlbumCoverCache resolves the path. Without the
+    // crossfade the cover snaps from the previous track's image to a
+    // slate placeholder to the new image — visible as a flicker.
+    final Widget coverChild;
     if (media.artUri != null) {
       // CachedNetworkImageProvider for HTTPS art URIs so the mini bar
       // hits the same disk cache the rest of the UI uses (ServerImage,
       // discover thumbnails). FileImage stays for the file:// branch
       // populated by AlbumCoverCache — bytes are already on disk and a
       // second cache layer would only burn duplicate space.
-      cover = Image(
+      coverChild = Image(
+        // Key includes the artUri so AnimatedSwitcher detects a swap;
+        // without this it treats successive Image widgets as the same
+        // tree node and skips the transition.
+        key: ValueKey('art-${media.artUri}'),
         image: media.artUri!.isScheme('file')
             ? FileImage(File.fromUri(media.artUri!)) as ImageProvider
             : CachedNetworkImageProvider(media.artUri.toString()),
@@ -117,8 +128,19 @@ class _TrackInfo extends StatelessWidget {
             Container(width: 48, height: 48, color: fs.slate),
       );
     } else {
-      cover = Container(width: 48, height: 48, color: fs.slate);
+      coverChild = Container(
+        key: const ValueKey('art-placeholder'),
+        width: 48,
+        height: 48,
+        color: fs.slate,
+      );
     }
+    final cover = AnimatedSwitcher(
+      duration: const Duration(milliseconds: 180),
+      switchInCurve: Curves.easeOut,
+      switchOutCurve: Curves.easeOut,
+      child: coverChild,
+    );
     return Row(
       // Default centering vertically aligns the like/kebab buttons
       // against the album art (48dp) — visually they span the title +