feat(flutter/cache): connectivityProvider + cacheSettingsProvider

connectivityProvider — StreamProvider<bool>; true when any connectivity
result is non-none. No Wi-Fi gate per operator decision.

cacheSettingsProvider — AsyncNotifier<CacheSettings> persisting
capBytes / prefetchWindow / cacheLikedTracks via flutter_secure_storage.
Defaults: 5 GB cap, 5-track prefetch, cache liked = on.

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

flutter_client/test/cache/connectivity_provider_test.dart

@@ -0,0 +1,14 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:minstrel/cache/connectivity_provider.dart';
+
+void main() {
+  test('connectivityProvider is reachable from a ProviderContainer', () {
+    // Smoke test: provider builds without throwing. Real connectivity
+    // tests would need a connectivity_plus mock; defer to manual ops.
+    final container = ProviderContainer();
+    addTearDown(container.dispose);
+    expect(container.read(connectivityProvider.future), isA<Future<bool>>());
+  });
+}