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/lib/cache/connectivity_provider.dart

@@ -0,0 +1,13 @@
+import 'package:connectivity_plus/connectivity_plus.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+/// Online if at least one connectivity result is non-none.
+/// connectivity_plus reports the union (wifi, mobile, vpn, etc.) — operator
+/// chose "no Wi-Fi gate" for #357, so any connection means "go ahead and
+/// pull/cache".
+final connectivityProvider = StreamProvider<bool>((ref) {
+  final c = Connectivity();
+  return c.onConnectivityChanged.map(
+    (results) => results.any((r) => r != ConnectivityResult.none),
+  );
+});