fix(flutter): full player kebab Go to album/artist navigates cleanly

Same root cause as the /queue duplicate-page-key crash: /now-playing
is a top-level route (lives outside the ShellRoute), but
/artists/:id and /albums/:id are shell-children. Pushing a shell-
child from a top-level route makes go_router attempt to mount a
second ShellRoute on top of the active one, leaving navigation in a
broken state. The mini player works because it's already inside the
shell.

Add an optional onBeforeNavigate callback to TrackActionsSheet
(forwarded through TrackActionsButton). When set, fires after
sheet.pop() and before context.push() of the destination route.

Wire the full player's TrackActionsButton with onBeforeNavigate:
() => Navigator.of(context).maybePop() so /now-playing dismisses
itself before the detail route is pushed. Result: clean navigation
into the destination, mini player visible underneath as expected.

Mini player keeps the default (no callback) since it's already in
the shell.

flutter_client/lib/player/now_playing_screen.dart

@@ -397,6 +397,12 @@ class _SecondaryControls extends StatelessWidget {
             streamUrl: '',
           ),
           hideQueueActions: true,
+          // /now-playing lives outside the ShellRoute. Pushing
+          // /artists/:id or /albums/:id (both shell-children) on top
+          // of it would make go_router try to mount a duplicate
+          // ShellRoute. Pop ourselves first so the destination route
+          // mounts cleanly inside the existing shell.
+          onBeforeNavigate: () => Navigator.of(context).maybePop(),
         ),
       ],
     );

flutter_client/lib/shared/widgets/track_actions/track_actions_button.dart

@@ -11,6 +11,7 @@ class TrackActionsButton extends StatelessWidget {
     super.key,
     required this.track,
     this.hideQueueActions = false,
+    this.onBeforeNavigate,
   });
 
   final TrackRef track;
@@ -19,6 +20,11 @@ class TrackActionsButton extends StatelessWidget {
   /// screen where the menu's track IS the currently-playing one.
   final bool hideQueueActions;
 
+  /// Forwarded to TrackActionsSheet.onBeforeNavigate. Use to dismiss
+  /// the host route before "Go to album" / "Go to artist" pushes the
+  /// detail screen.
+  final void Function()? onBeforeNavigate;
+
   @override
   Widget build(BuildContext context) {
     final fs = Theme.of(context).extension<FabledSwordTheme>()!;
@@ -29,6 +35,7 @@ class TrackActionsButton extends StatelessWidget {
         context,
         track,
         hideQueueActions: hideQueueActions,
+        onBeforeNavigate: onBeforeNavigate,
       ),
     );
   }

flutter_client/lib/shared/widgets/track_actions/track_actions_sheet.dart

@@ -21,15 +21,25 @@ class TrackActionsSheet extends ConsumerWidget {
     super.key,
     required this.track,
     required this.hideQueueActions,
+    this.onBeforeNavigate,
   });
 
   final TrackRef track;
   final bool hideQueueActions;
 
+  /// Optional hook invoked just before "Go to album" / "Go to artist"
+  /// pushes the destination route. Lets callers that live OUTSIDE the
+  /// ShellRoute (e.g. the full player at /now-playing) pop themselves
+  /// first, so go_router doesn't try to mount a second ShellRoute on
+  /// top of the active top-level route — the same duplicate-page-key
+  /// failure mode we hit when /queue lived inside the shell.
+  final void Function()? onBeforeNavigate;
+
   static Future<void> show(
     BuildContext context,
     TrackRef track, {
     bool hideQueueActions = false,
+    void Function()? onBeforeNavigate,
   }) {
     return showModalBottomSheet<void>(
       context: context,
@@ -37,6 +47,7 @@ class TrackActionsSheet extends ConsumerWidget {
       builder: (_) => TrackActionsSheet(
         track: track,
         hideQueueActions: hideQueueActions,
+        onBeforeNavigate: onBeforeNavigate,
       ),
     );
   }
@@ -141,6 +152,7 @@ class TrackActionsSheet extends ConsumerWidget {
               label: 'Go to album',
               onTap: () {
                 Navigator.pop(context);
+                onBeforeNavigate?.call();
                 context.push('/albums/${track.albumId}');
               },
             ),
@@ -150,6 +162,7 @@ class TrackActionsSheet extends ConsumerWidget {
               label: 'Go to artist',
               onTap: () {
                 Navigator.pop(context);
+                onBeforeNavigate?.call();
                 context.push('/artists/${track.artistId}');
               },
             ),