feat(chat): 发收消息全量实现 (#25~#28)

- 移除 @riverpod/@freezed 注解依赖，全部改为手写 Provider（无需 build_runner）
  · LoginState 改为纯 Dart，LoginViewModel/ThemeViewModel/ChatViewModel 改为 Notifier
  · UserNotifier 改为 FamilyAsyncNotifier<User?,int>，mini_app_provider 改为手写 Provider
  · 15 个 StreamProvider/StreamProvider.family 从 @riverpod 迁移至手写

- 发送消息（#25）
  · SendMessageRequest/SendMessageResponse DTO
  · SendMessageUseCase：乐观写入 DB → HTTP POST → 更新 Chat 摘要

- 接收消息 WS（#26）
  · WsMessageService：监听 mode2 WS 帧 → HTTP 补拉 → DB 写入 → Chat 更新
  · FetchHistoryRequest/FetchHistoryResponse DTO（GET /app/api/chat/history）
  · FetchHistoryUseCase：拉取 → insertOrReplaceAll

- DI 装配（chat_service_providers.dart）
  · wsMessageServiceProvider、sendMessageUseCaseProvider、fetchHistoryUseCaseProvider

- 聊天列表页（#27）
  · ChatListViewModel（Notifier<void>）+ chat_page.dart 真实会话列表 UI
  · ListTile：头像首字母、最新消息摘要、未读角标、时间格式化

- 聊天详情页（#28）
  · ChatDetailViewModel（FamilyNotifier<ChatDetailState,int>）+ chat_detail_page.dart
  · 消息气泡（自己/他人分左右）、底部输入框、发送状态与错误提示

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/im_app/lib/features/chat/usecases/fetch_history_use_case.dart

@@ -0,0 +1,42 @@
+import 'package:networks_sdk/networks_sdk.dart';
+
+import 'package:im_app/data/remote/fetch_history_request.dart';
+import 'package:im_app/domain/repositories/message_repository.dart';
+
+/// 拉取消息历史用例
+///
+/// 调用 HTTP GET `/app/api/chat/history`，将消息写入本地 DB。
+/// DB Stream → StreamProvider → UI 自动重建。
+///
+/// ## 分页
+///
+/// [chatIdx] 为锚点消息 index：
+/// - 0 → 拉最新 [limit] 条
+/// - N → 拉 idx < N 的 [limit] 条（向上翻页）
+///
+/// 返回拉取到的消息数，0 表示已到头或网络失败。
+class FetchHistoryUseCase {
+  final NetworksSdkApi _apiClient;
+  final MessageRepository _messageRepo;
+
+  FetchHistoryUseCase({
+    required NetworksSdkApi apiClient,
+    required MessageRepository messageRepo,
+  })  : _apiClient = apiClient,
+        _messageRepo = messageRepo;
+
+  Future<int> execute({
+    required int chatId,
+    int chatIdx = 0,
+    int limit = 20,
+  }) async {
+    final response = await _apiClient.executeRequest(
+      FetchHistoryRequest(chatId: chatId, chatIdx: chatIdx, limit: limit),
+    );
+    if (response == null || response.messages.isEmpty) return 0;
+
+    final entities = response.messages.map((m) => m.toEntity()).toList();
+    await _messageRepo.insertOrReplaceAll(entities);
+    return entities.length;
+  }
+}

apps/im_app/lib/features/chat/usecases/send_message_use_case.dart

@@ -0,0 +1,83 @@
+import 'package:flutter/foundation.dart';
+import 'package:networks_sdk/networks_sdk.dart';
+
+import 'package:im_app/data/remote/send_message_request.dart';
+import 'package:im_app/domain/entities/message.dart';
+import 'package:im_app/domain/repositories/chat_repository.dart';
+import 'package:im_app/domain/repositories/message_repository.dart';
+
+/// 发送文本消息用例
+///
+/// ## 执行流程
+/// 1. 乐观写入本地 DB（临时消息，id=0）→ UI 立即刷新
+/// 2. HTTP POST `/app/api/chat/send-message` → 获取服务端 messageId / chatIdx
+/// 3. 更新 ChatRepository 的 lastMsg / lastTyp / lastTime
+///
+/// DB Stream → StreamProvider → UI 自动重建，无需额外通知。
+class SendMessageUseCase {
+  final NetworksSdkApi _apiClient;
+  final MessageRepository _messageRepo;
+  final ChatRepository _chatRepo;
+  final int currentUid;
+
+  SendMessageUseCase({
+    required NetworksSdkApi apiClient,
+    required MessageRepository messageRepo,
+    required ChatRepository chatRepo,
+    required this.currentUid,
+  })  : _apiClient = apiClient,
+        _messageRepo = messageRepo,
+        _chatRepo = chatRepo;
+
+  Future<void> execute({
+    required int chatId,
+    required String content,
+    int typ = 1,
+  }) async {
+    final sendTime = DateTime.now().millisecondsSinceEpoch ~/ 1000;
+
+    // 1. 乐观本地写入
+    await _messageRepo.insertOrReplace(
+      Message(
+        id: 0,
+        chatId: chatId,
+        sendId: currentUid,
+        content: content,
+        typ: typ,
+        sendTime: sendTime,
+      ),
+    );
+
+    // 2. HTTP 发送
+    SendMessageResponse? resp;
+    try {
+      resp = await _apiClient.executeRequest(
+        SendMessageRequest(
+          chatId: chatId,
+          content: content,
+          typ: typ,
+          sendTime: sendTime,
+        ),
+      );
+    } catch (e) {
+      debugPrint('[SendMessageUseCase] HTTP error: $e');
+    }
+
+    // 3. 更新 Chat 摘要
+    try {
+      final chat = await _chatRepo.getChat(chatId);
+      if (chat != null) {
+        await _chatRepo.updateChat(
+          chat.copyWith(
+            lastMsg: content,
+            lastTyp: typ,
+            lastTime: sendTime,
+            msgIdx: resp?.chatIdx ?? chat.msgIdx,
+          ),
+        );
+      }
+    } catch (e) {
+      debugPrint('[SendMessageUseCase] updateChat error: $e');
+    }
+  }
+}