feat(app): 发现页动态化 — DiscoverBloc + API 驱动 + 下拉刷新

- 新增 DiscoverBloc (LoadData/Refresh) + DiscoverModels 4 个数据类
- DiscoverPage 改为 BlocBuilder 驱动: loading/loaded/error/empty 四态
- 替换全部硬编码占位数据为 API 实时数据
- 添加 RefreshIndicator 下拉刷新
- 离线异常时保留已有数据，友好错误提示

app/lib/features/discover/bloc/discover_bloc.dart

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+// 发现页 BLoC — 管理发现页数据加载状态
+//
+// 职责：调用 /diary/discover API，解析响应，管理加载/成功/失败状态。
+
+import 'package:flutter_bloc/flutter_bloc.dart';
+
+import '../../../data/remote/api_client.dart';
+import '../models/discover_models.dart';
+
+part 'discover_event.dart';
+part 'discover_state.dart';
+
+class DiscoverBloc extends Bloc<DiscoverEvent, DiscoverState> {
+  final ApiClient _api;
+
+  DiscoverBloc({required ApiClient api})
+      : _api = api,
+        super(const DiscoverInitial()) {
+    on<DiscoverLoadData>(_onLoadData);
+    on<DiscoverRefresh>(_onRefresh);
+  }
+
+  /// 首次加载 — 显示 loading 状态
+  Future<void> _onLoadData(
+    DiscoverLoadData event,
+    Emitter<DiscoverState> emit,
+  ) async {
+    emit(const DiscoverLoading());
+    await _fetchData(emit);
+  }
+
+  /// 刷新 — 不显示 loading，静默更新
+  Future<void> _onRefresh(
+    DiscoverRefresh event,
+    Emitter<DiscoverState> emit,
+  ) async {
+    await _fetchData(emit);
+  }
+
+  /// 通用数据获取逻辑
+  Future<void> _fetchData(Emitter<DiscoverState> emit) async {
+    try {
+      final response = await _api.get('/diary/discover');
+      final body = response.data as Map<String, dynamic>;
+
+      // 后端信封格式: { success, data: { ... }, message }
+      final dataJson = body['data'] as Map<String, dynamic>? ?? {};
+      final discoverData = DiscoverData.fromJson(dataJson);
+
+      emit(DiscoverLoaded(discoverData));
+    } on OfflineException {
+      // 离线时，如果有已加载的数据，保留它
+      if (state is DiscoverLoaded) return;
+      emit(const DiscoverError('网络不可用，请检查网络连接'));
+    } catch (e) {
+      if (state is DiscoverLoaded) return;
+      emit(DiscoverError('加载失败：${_friendlyError(e)}'));
+    }
+  }
+
+  /// 将异常转换为用户友好的错误消息
+  String _friendlyError(Object error) {
+    final msg = error.toString();
+    if (msg.contains('SocketException') || msg.contains('Connection refused')) {
+      return '无法连接服务器';
+    }
+    if (msg.contains('401')) {
+      return '登录已过期，请重新登录';
+    }
+    if (msg.contains('403')) {
+      return '没有访问权限';
+    }
+    if (msg.contains('500')) {
+      return '服务器错误，请稍后重试';
+    }
+    return '请稍后重试';
+  }
+}

app/lib/features/discover/bloc/discover_event.dart

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+part of 'discover_bloc.dart';
+
+/// 发现页事件
+sealed class DiscoverEvent {
+  const DiscoverEvent();
+}
+
+/// 加载发现页数据（首次进入或重新进入页面）
+final class DiscoverLoadData extends DiscoverEvent {
+  const DiscoverLoadData();
+}
+
+/// 下拉刷新（不显示全屏 loading，避免闪烁）
+final class DiscoverRefresh extends DiscoverEvent {
+  const DiscoverRefresh();
+}

app/lib/features/discover/bloc/discover_state.dart

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+part of 'discover_bloc.dart';
+
+/// 发现页状态
+sealed class DiscoverState {
+  const DiscoverState();
+}
+
+/// 初始状态
+final class DiscoverInitial extends DiscoverState {
+  const DiscoverInitial();
+}
+
+/// 加载中
+final class DiscoverLoading extends DiscoverState {
+  const DiscoverLoading();
+}
+
+/// 加载成功
+final class DiscoverLoaded extends DiscoverState {
+  final DiscoverData data;
+
+  const DiscoverLoaded(this.data);
+}
+
+/// 加载失败
+final class DiscoverError extends DiscoverState {
+  final String message;
+
+  const DiscoverError(this.message);
+}