fix(flutter): full-player kebab nav + restore artistAlbums SWR

Two unrelated issues, batched.

Full-player kebab → "Go to album/artist" still crashed with
_debugCheckDuplicatedPageKeys despite the prior onBeforeNavigate
fix. Cause: pop() and push() ran in the same frame, so go_router's
page-key reservation table briefly contained both /now-playing AND
the destination shell-child, tripping the duplicate-key assert.

Refactor: replace onBeforeNavigate with onNavigate(path), where the
host receives the path AFTER the sheet pops and owns the
navigation entirely. The full player wires:
  onNavigate: (path) async {
    await Navigator.of(context).maybePop();
    if (context.mounted) GoRouter.of(context).push(path);
  }
Awaiting the pop guarantees /now-playing is fully gone from the
navigator's page list before the push starts.

Mini player + track rows + everywhere else use the default (no
onNavigate) path that does context.push(path) inline — they're
already inside the ShellRoute, no race possible.

Restored alwaysRefresh: true on artistAlbumsProvider. Dropping it
in the cache-loop fix had a side effect: if drift's cachedAlbums
held only a subset of an artist's albums (user previously visited
just one album by them), the artist detail page rendered that
partial list forever — provider only fetched on empty drift, never
on partial drift. The metadata prefetcher only mass-warms
artistProvider (single row), so re-enabling SWR on artistAlbums
won't recreate the storm.

flutter_client/lib/player/now_playing_screen.dart

@@ -400,9 +400,14 @@ class _SecondaryControls extends StatelessWidget {
           // /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(),
+          // ShellRoute (the same _debugCheckDuplicatedPageKeys crash
+          // we hit before with /queue). Await our own pop fully
+          // before pushing the destination so go_router never sees
+          // both pages active in the same frame.
+          onNavigate: (path) async {
+            await Navigator.of(context).maybePop();
+            if (context.mounted) GoRouter.of(context).push(path);
+          },
         ),
       ],
     );