Refactor unread message tracking and implement channel caching (#126)

* Refactor unread message tracking and implement channel caching

* formatted files

lib/storage/channel_store.dart

@@ -0,0 +1,50 @@
+import 'dart:convert';
+import 'dart:typed_data';
+
+import '../models/channel.dart';
+import 'prefs_manager.dart';
+
+class ChannelStore {
+  static const String _key = 'channels';
+
+  Future<List<Channel>> loadChannels() async {
+    final prefs = PrefsManager.instance;
+    final jsonStr = prefs.getString(_key);
+    if (jsonStr == null) return [];
+
+    try {
+      final jsonList = jsonDecode(jsonStr) as List<dynamic>;
+      return jsonList
+          .map((entry) => _fromJson(entry as Map<String, dynamic>))
+          .toList();
+    } catch (_) {
+      return [];
+    }
+  }
+
+  Future<void> saveChannels(List<Channel> channels) async {
+    final prefs = PrefsManager.instance;
+    final jsonList = channels.map(_toJson).toList();
+    await prefs.setString(_key, jsonEncode(jsonList));
+  }
+
+  Map<String, dynamic> _toJson(Channel channel) {
+    return {
+      'index': channel.index,
+      'name': channel.name,
+      'psk': base64Encode(channel.psk),
+      'unreadCount': channel.unreadCount,
+    };
+  }
+
+  Channel _fromJson(Map<String, dynamic> json) {
+    return Channel(
+      index: json['index'] as int,
+      name: json['name'] as String? ?? '',
+      psk: json['psk'] != null
+          ? Uint8List.fromList(base64Decode(json['psk'] as String))
+          : Uint8List(16),
+      unreadCount: json['unreadCount'] as int? ?? 0,
+    );
+  }
+}