feat(app): 数据层集成 — RemoteJournalRepository + ClassRepository + SSE 通知

数据层新增:
- RemoteJournalRepository: 日记 CRUD + 元素管理，通过 ApiClient 连接后端
- ClassRepository: 班级/主题/评语 API 操作（getMyClasses/joinClass/assignTopic/createComment）
- SseNotificationService: SSE 实时通知监听 + 自动重连 + 事件流
- ApiException: 统一 API 错误封装
- DTO: ClassMemberDto + TopicDto + CommentDto

设计:
- Repository 模式: 抽象接口 + 远程实现 + 内存实现
- SSE: Dio stream + SSE 协议解析 + 3秒自动重连
- 所有 Repository 通过 ApiClient 注入，依赖现有 JWT 拦截器

验证: flutter analyze 0 error

app/lib/data/services/sse_notification_service.dart

@@ -0,0 +1,146 @@
+// SSE 通知服务 — 监听服务端推送事件
+
+import 'dart:async';
+import 'dart:convert';
+
+import 'package:dio/dio.dart';
+
+/// SSE 通知事件
+class NotificationEvent {
+  final String type;
+  final Map<String, dynamic> payload;
+  final DateTime receivedAt;
+
+  const NotificationEvent({
+    required this.type,
+    required this.payload,
+    required this.receivedAt,
+  });
+}
+
+/// SSE 通知服务 — 监听后端 Server-Sent Events 推送
+///
+/// 使用方式：
+/// ```dart
+/// final service = SseNotificationService(token: 'jwt-token');
+/// service.events.listen((event) {
+///   // 处理通知
+/// });
+/// await service.connect();
+/// ```
+class SseNotificationService {
+  final String _baseUrl;
+  final String? _token;
+
+  Dio? _dio;
+  Response<ResponseBody>? _response;
+  StreamController<NotificationEvent>? _controller;
+  bool _disposed = false;
+
+  SseNotificationService({
+    required String token,
+    String baseUrl = 'http://localhost:8080/api/v1',
+  })  : _token = token,
+        _baseUrl = baseUrl;
+
+  /// 通知事件流
+  Stream<NotificationEvent> get events {
+    _controller ??= StreamController<NotificationEvent>.broadcast();
+    return _controller!.stream;
+  }
+
+  /// 连接到 SSE 端点
+  Future<void> connect() async {
+    if (_disposed) return;
+
+    _dio = Dio(BaseOptions(
+      baseUrl: _baseUrl,
+      headers: {
+        'Accept': 'text/event-stream',
+        'Cache-Control': 'no-cache',
+        if (_token != null) 'Authorization': 'Bearer $_token',
+      },
+      responseType: ResponseType.stream,
+    ));
+
+    try {
+      _response = await _dio!.get<ResponseBody>('/message/stream');
+
+      if (_response?.data == null) return;
+
+      _response!.data!.stream.listen(
+        (data) {
+          _parseSseData(data);
+        },
+        onError: (error) {
+          if (!_disposed) {
+            // 自动重连逻辑（3秒延迟）
+            Future.delayed(const Duration(seconds: 3), () {
+              if (!_disposed) connect();
+            });
+          }
+        },
+        onDone: () {
+          if (!_disposed) {
+            Future.delayed(const Duration(seconds: 3), () {
+              if (!_disposed) connect();
+            });
+          }
+        },
+      );
+    } catch (e) {
+      // 连接失败，延迟重连
+      if (!_disposed) {
+        Future.delayed(const Duration(seconds: 5), () {
+          if (!_disposed) connect();
+        });
+      }
+    }
+  }
+
+  /// 解析 SSE 数据帧
+  void _parseSseData(List<int> data) {
+    final text = utf8.decode(data);
+    final lines = text.split('\n');
+
+    String? eventType;
+    String? eventData;
+
+    for (final line in lines) {
+      if (line.startsWith('event:')) {
+        eventType = line.substring(6).trim();
+      } else if (line.startsWith('data:')) {
+        eventData = line.substring(5).trim();
+      } else if (line.isEmpty && eventType != null && eventData != null) {
+        // 空行 = 事件结束
+        _emitEvent(eventType, eventData);
+        eventType = null;
+        eventData = null;
+      }
+    }
+  }
+
+  /// 发射通知事件到流
+  void _emitEvent(String type, String data) {
+    if (_disposed || _controller == null) return;
+
+    try {
+      final payload = jsonDecode(data) as Map<String, dynamic>;
+      _controller!.add(NotificationEvent(
+        type: type,
+        payload: payload,
+        receivedAt: DateTime.now(),
+      ));
+    } catch (_) {
+      // 忽略解析错误
+    }
+  }
+
+  /// 断开连接并释放资源
+  void dispose() {
+    _disposed = true;
+    _response?.data?.stream.listen((_) {});
+    _controller?.close();
+    _dio?.close();
+  }
+}