fix(flutter): clear analyze --fatal-infos issues

Three info-level lints from CI:

- playlist_card.dart doc comment: "/playlists/<id>" → "/playlists/{id}"
  in backticks, since `<id>` was flagged as unintended HTML.
- compact_track_card_test.dart: home: Scaffold(...) is now const, which
  propagates const to CompactTrackCard and makes the inner [_track]
  implicitly const.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/test/library/widgets/compact_track_card_test.dart

@@ -23,10 +23,10 @@ void main() {
     await tester.pumpWidget(ProviderScope(
       child: MaterialApp(
         theme: buildThemeData(),
-        home: Scaffold(
+        home: const Scaffold(
           body: CompactTrackCard(
             track: _track,
-            sectionTracks: const [_track],
+            sectionTracks: [_track],
             index: 0,
           ),
         ),