feat: endless mode UI - game over card, best score, HUD

lib/state/endless_best_notifier.dart

@@ -0,0 +1,18 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import 'providers.dart';
+
+/// Reactive view over SaveRepository's endless best score.
+class EndlessBestNotifier extends Notifier<int> {
+  @override
+  int build() => ref.read(saveRepositoryProvider).endlessBest;
+
+  /// Records the run; returns true when it set a new best.
+  Future<bool> record(int score) async {
+    final repo = ref.read(saveRepositoryProvider);
+    final isNewBest = score > state;
+    await repo.recordEndlessScore(score);
+    state = repo.endlessBest;
+    return isNewBest;
+  }
+}