fix(flutter): /queue at top level — fixes duplicate-key crash from player

Tapping the queue button from /now-playing crashed with
NavigatorState._debugCheckDuplicatedPageKeys. Root cause: when I
moved /now-playing out of the ShellRoute earlier, /queue was left
inside the shell. Pushing /queue while /now-playing was on top
made go_router try to mount a second ShellRoute instance under the
existing one — both shells got the same page key.

Move /queue out of the ShellRoute too, sibling to /now-playing.
Shell stays mounted (with whatever child it had) underneath both
top-level routes; pop from /queue returns to /now-playing or the
shell's previous child as appropriate.

flutter_client/lib/shared/routing.dart

@@ -85,6 +85,14 @@ GoRouter buildRouter(Ref ref) {
           },
         ),
       ),
+      // /queue lives outside the ShellRoute too. Why: pushing /queue
+      // from /now-playing (which is also outside the shell) used to
+      // cause go_router to mount a second ShellRoute instance under
+      // the existing one, producing a duplicate page-key assertion
+      // (NavigatorState._debugCheckDuplicatedPageKeys). Top-level
+      // routes can stack on each other freely; shell-children can't
+      // when something on top of the shell is already routing.
+      GoRoute(path: '/queue', builder: (_, __) => const QueueScreen()),
       ShellRoute(
         builder: (ctx, state, child) => VersionGate(child: _ShellWithPlayerBar(child: child)),
         routes: [
@@ -105,7 +113,6 @@ GoRouter buildRouter(Ref ref) {
               seed: s.extra is AlbumRef ? s.extra as AlbumRef : null,
             ),
           ),
-          GoRoute(path: '/queue', builder: (_, __) => const QueueScreen()),
           GoRoute(path: '/search', builder: (_, __) => const SearchScreen()),
           GoRoute(path: '/library', builder: (_, __) => const LibraryScreen()),
           GoRoute(path: '/discover', builder: (_, __) => const DiscoverScreen()),