fix(flutter): clear analyze --fatal-infos errors from admin parity slice

CI caught six issues against the new admin slice:

- AsyncValue<T> in this Riverpod 3.3.1 codebase exposes `.value`
  (returns T?), not `.valueOrNull`. Switched the three admin readers
  to match the established convention (player_bar, queue_screen,
  now_playing_screen all use `.value`).
- home_screen.dart still has ctx.push calls in _albumsRow / _artistsRow
  (carousel tap handlers) — restored the go_router import that
  Task 2 over-eagerly removed.
- admin_user_edit_sheet.dart had a single-line `if (!await ...) return;`
  that violated curly_braces_in_flow_control_structures. Wrapped in
  braces.
- admin_quarantine_item.dart doc comment had `<top>` placeholders that
  the analyzer flagged as unintended HTML. Rephrased without angle
  brackets.

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

flutter_client/lib/models/admin_quarantine_item.dart

@@ -55,9 +55,9 @@ class AdminQuarantineItem {
 
   final List<AdminQuarantineReport> reports;
 
-  /// One-line summary of the most-cited reason. Returns "<top>" when
-  /// only one reason exists or "<top> (+N more)" when there are
-  /// additional distinct reasons.
+  /// One-line summary of the most-cited reason. Returns just the top
+  /// reason when only one reason exists, or `top (+N more)` when there
+  /// are additional distinct reasons.
   String get topReasonSummary {
     if (reasonCounts.isEmpty) return '';
     final entries = reasonCounts.entries.toList()