fix(flutter): player bar layout, system playlist tap, banner height

Player bar: drop the volume slider — phone hardware controls volume.
Right cluster collapses to a single row of shuffle/repeat/queue.

Playlist card: GestureDetector defers to children by default, so taps
over the cover image area didn't always register. Add HitTestBehavior.opaque
so the whole card surface is tappable.

Update banner: was 3-4× the height of its text — IconButton's default
48dp tap target plus generous vertical padding dominated. Tighten
padding (8→4), shrink dismiss button to 32×32 with iconSize 16, slim
the action button to 28dp height with shrinkWrap tap target.

Home screen: ClampingScrollPhysics + 140dp bottom padding (already
applied in working tree) so scroll stops just above the mini-player.

flutter_client/lib/playlists/widgets/playlist_card.dart

@@ -19,6 +19,7 @@ class PlaylistCard extends StatelessWidget {
     return SizedBox(
       width: 176,
       child: GestureDetector(
+        behavior: HitTestBehavior.opaque,
         onTap: () => context.push('/playlists/${playlist.id}'),
         child: Padding(
           padding: const EdgeInsets.symmetric(horizontal: 8),