feat(flutter): mini ↔ full player transition + flesh out NowPlayingScreen

Mini player simplifies to track-info + prev/play/next. The
shuffle/repeat/queue cluster moves to the full player, where it has
the room to breathe.

Full player (NowPlayingScreen) now has:
- Real album art (FileImage when artUri is file://, ServerImage from
  /api/albums/<id>/cover otherwise — same auth-aware loader as the
  rest of the app, with errorBuilder so a failed fetch falls back to
  a slate placeholder instead of a broken-image glyph).
- Title row with inline like + kebab.
- Full-width seek with start/end timestamps below.
- 36/72/36 prev/play/next.
- Secondary row: shuffle, repeat, queue.

Mini ↔ Full transition mechanics:
- Tap mini → push /now-playing with custom slide-up transition
  (CustomTransitionPage, 280ms easeOutCubic).
- Vertical drag-up flick on mini (>200 px/s) → same push.
- System back / leading expand-more icon → Navigator.pop, which the
  CustomTransitionPage replays as a slide-down (240ms easeInCubic).
- Drag-down on full player past 80px OR a >500 px/s downward flick
  → Navigator.pop. Buttons and the seek slider stay tappable since
  Flutter's gesture arena gives priority to the more-specific child
  recognizers.
- Mini player hides while /now-playing is the active route — the
  full player IS the player UI in that state, no need to fight over
  visual space.

flutter_client/lib/shared/routing.dart

@@ -67,7 +67,32 @@ GoRouter buildRouter(Ref ref) {
             path: '/albums/:id',
             builder: (_, s) => AlbumDetailScreen(id: s.pathParameters['id']!),
           ),
-          GoRoute(path: '/now-playing', builder: (_, __) => const NowPlayingScreen()),
+          GoRoute(
+            path: '/now-playing',
+            // Slide-up transition: full player ascends from the bottom
+            // edge (matching where the mini player sits) into a full
+            // screen view. Drag-down dismissal in NowPlayingScreen
+            // mirrors this with a slide back down via Navigator.pop.
+            pageBuilder: (_, __) => CustomTransitionPage(
+              child: const NowPlayingScreen(),
+              transitionDuration: const Duration(milliseconds: 280),
+              reverseTransitionDuration: const Duration(milliseconds: 240),
+              transitionsBuilder: (_, anim, __, child) {
+                final eased = CurvedAnimation(
+                  parent: anim,
+                  curve: Curves.easeOutCubic,
+                  reverseCurve: Curves.easeInCubic,
+                );
+                return SlideTransition(
+                  position: Tween(
+                    begin: const Offset(0, 1),
+                    end: Offset.zero,
+                  ).animate(eased),
+                  child: child,
+                );
+              },
+            ),
+          ),
           GoRoute(path: '/queue', builder: (_, __) => const QueueScreen()),
           GoRoute(path: '/search', builder: (_, __) => const SearchScreen()),
           GoRoute(path: '/library', builder: (_, __) => const LibraryScreen()),
@@ -101,6 +126,11 @@ class _ShellWithPlayerBar extends ConsumerWidget {
     // When the banner is showing, strip that inset from the child so
     // its AppBar lands directly under the banner.
     final hasBanner = ref.watch(shouldShowUpdateBannerProvider) != null;
+    // Hide the mini player when the full player is on top — the full
+    // player IS the player UI in that state, and overlapping them
+    // would just fight for visual space.
+    final loc = GoRouterState.of(context).matchedLocation;
+    final hideMini = loc == '/now-playing';
     return Column(
       children: [
         const UpdateBanner(),
@@ -113,7 +143,7 @@ class _ShellWithPlayerBar extends ConsumerWidget {
                 )
               : child,
         ),
-        const PlayerBar(),
+        if (!hideMini) const PlayerBar(),
       ],
     );
   }