feat(chat): add ChatHistoryRepository with shared_preferences

apps/lib/features/chat/data/repositories/chat_history_repository.dart

@@ -0,0 +1,62 @@
+import 'dart:convert';
+import 'package:shared_preferences/shared_preferences.dart';
+
+class ChatHistoryRepository {
+  static const String _messagesKey = 'chat_messages_';
+  static const String _lastRunIdKey = 'chat_last_run_id_';
+  static const String _calendarEventsKey = 'calendar_events';
+
+  final String threadId;
+
+  ChatHistoryRepository({this.threadId = 'default'});
+
+  String get _msgKey => '$_messagesKey$threadId';
+  String get _runIdKey => '$_lastRunIdKey$threadId';
+
+  Future<void> saveMessages(List<Map<String, dynamic>> messages) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_msgKey, jsonEncode(messages));
+  }
+
+  Future<List<Map<String, dynamic>>?> loadMessages() async {
+    final prefs = await SharedPreferences.getInstance();
+    final data = prefs.getString(_msgKey);
+    if (data == null) return null;
+    final list = jsonDecode(data) as List;
+    return list.cast<Map<String, dynamic>>();
+  }
+
+  Future<void> saveLastRunId(String runId) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_runIdKey, runId);
+  }
+
+  Future<String?> loadLastRunId() async {
+    final prefs = await SharedPreferences.getInstance();
+    return prefs.getString(_runIdKey);
+  }
+
+  Future<void> clear() async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.remove(_msgKey);
+    await prefs.remove(_runIdKey);
+  }
+
+  Future<void> saveCalendarEvent(Map<String, dynamic> event) async {
+    final prefs = await SharedPreferences.getInstance();
+    final eventsJson = prefs.getString(_calendarEventsKey);
+    final events = eventsJson != null
+        ? jsonDecode(eventsJson) as Map<String, dynamic>
+        : <String, dynamic>{};
+    events[event['id']] = event;
+    await prefs.setString(_calendarEventsKey, jsonEncode(events));
+  }
+
+  Future<List<Map<String, dynamic>>> loadCalendarEvents() async {
+    final prefs = await SharedPreferences.getInstance();
+    final eventsJson = prefs.getString(_calendarEventsKey);
+    if (eventsJson == null) return [];
+    final events = jsonDecode(eventsJson) as Map<String, dynamic>;
+    return events.values.cast<Map<String, dynamic>>().toList();
+  }
+}