m3u: Add disk cache persistence and immediate cached-return + background refresh

lib/providers/m3u_provider.dart

@@ -1,4 +1,5 @@
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter/foundation.dart' show debugPrint;
 import '../models/stream_entry.dart';
 import '../services/m3u_service.dart';
 
@@ -18,6 +19,22 @@ class ChannelListNotifier extends StateNotifier<AsyncValue<List<StreamEntry>>> {
   ChannelListNotifier(this._svc) : super(const AsyncValue.data([]));
 
   Future<void> fetch(String url, {bool force = false}) async {
+    // If not forcing, try to load a persistent cache first and return it immediately
+    if (!force) {
+      try {
+        final cached = await _svc.loadFromCache(url);
+        if (cached != null) {
+          state = AsyncValue.data(cached);
+          // Refresh in background to get fresh results
+          _refreshInBackground(url);
+          return;
+        }
+      } catch (e) {
+        // ignore cache read errors and fall back to network fetch
+        debugPrint('Cache read failed during fetch: $e');
+      }
+    }
+
     state = const AsyncValue.loading();
     try {
       final entries = await _svc.fetch(url, forceRefresh: force);
@@ -27,5 +44,16 @@ class ChannelListNotifier extends StateNotifier<AsyncValue<List<StreamEntry>>> {
     }
   }
 
+  Future<void> _refreshInBackground(String url) async {
+    try {
+      final entries = await _svc.fetch(url, forceRefresh: true);
+      // Only update state if still not error or older content
+      state = AsyncValue.data(entries);
+    } catch (e) {
+      debugPrint('Background refresh failed: $e');
+      // keep cached data; don't overwrite with error
+    }
+  }
+
   Future<void> refresh(String url) async => fetch(url, force: true);
 }