feat(app): 设置页 UI + Mood/成就/贴纸 BLoC 接入 API + B7 测试扩展

前端改动:
- 新建设置页面 (主题切换/关于/隐私政策/用户协议/儿童隐私保护)
- SettingsBloc 注册到 MultiRepositoryProvider 全局可访问
- MoodBloc 修复编译错误 + 接入 /diary/stats/mood API
- MoodPage 添加错误状态展示和重试按钮
- AchievementBloc + 页面改造接入 /diary/achievements API
- StickerBloc + 页面改造接入 /diary/sticker-packs API
- TemplateBloc + 页面改造接入 /diary/templates API
- ProfilePage 设置入口改为跳转 /settings
- 添加 /settings 路由

后端改动:
- 扩展 mood_stats_service 测试 (连续天数算法/心情计数/边界场景)
- 新增 class_service 测试 (班级码生成/唯一性/错误映射)
- 新增 achievement_service 测试 (DTO 结构/序列化/map 构建)
- 新增 sticker_service 测试 (DTO 序列化/错误处理)
- 扩展 dto.rs 测试 (achievement/mood_stats/sticker/template/notification)
- 清理 2 个 unused import warning

验证:
- cargo check 0 error 0 warning
- flutter analyze 0 error

app/lib/features/achievement/bloc/achievement_bloc.dart

@@ -0,0 +1,108 @@
+// 成就 BLoC — 通过 API 加载成就列表
+
+import 'package:flutter/material.dart';
+import 'package:nuanji_app/data/remote/api_client.dart';
+
+// ===== 模型 =====
+
+/// 成就数据
+class Achievement {
+  final String id;
+  final String code;
+  final String name;
+  final String? description;
+  final String? icon;
+  final String category;
+  final bool isUnlocked;
+  final DateTime? unlockedAt;
+
+  const Achievement({
+    required this.id,
+    required this.code,
+    required this.name,
+    this.description,
+    this.icon,
+    required this.category,
+    this.isUnlocked = false,
+    this.unlockedAt,
+  });
+}
+
+// ===== State =====
+
+/// 成就页面状态
+class AchievementState {
+  final List<Achievement> achievements;
+  final bool isLoading;
+  final String? errorMessage;
+
+  const AchievementState({
+    this.achievements = const [],
+    this.isLoading = false,
+    this.errorMessage,
+  });
+
+  int get unlockedCount =>
+      achievements.where((a) => a.isUnlocked).length;
+
+  AchievementState copyWith({
+    List<Achievement>? achievements,
+    bool? isLoading,
+    String? errorMessage,
+  }) =>
+      AchievementState(
+        achievements: achievements ?? this.achievements,
+        isLoading: isLoading ?? this.isLoading,
+        errorMessage: errorMessage,
+      );
+}
+
+// ===== BLoC =====
+
+/// 成就 BLoC — ChangeNotifier 模式
+class AchievementBloc extends ChangeNotifier {
+  final ApiClient _api;
+  AchievementState _state = const AchievementState();
+  AchievementState get state => _state;
+
+  AchievementBloc({required ApiClient api}) : _api = api;
+
+  /// 加载成就列表
+  void load() {
+    _state = _state.copyWith(isLoading: true);
+    notifyListeners();
+    _fetchAchievements();
+  }
+
+  Future<void> _fetchAchievements() async {
+    try {
+      final response = await _api.get('/diary/achievements');
+      final body = response.data as Map<String, dynamic>;
+      final list = body['data'] as List? ?? [];
+
+      final achievements = list.map((item) {
+        final m = item as Map<String, dynamic>;
+        return Achievement(
+          id: m['id'] as String,
+          code: m['code'] as String,
+          name: m['name'] as String,
+          description: m['description'] as String?,
+          icon: m['icon'] as String?,
+          category: m['category'] as String,
+          isUnlocked: m['is_unlocked'] as bool? ?? false,
+          unlockedAt: m['unlocked_at'] != null
+              ? DateTime.tryParse(m['unlocked_at'] as String)
+              : null,
+        );
+      }).toList();
+
+      _state = _state.copyWith(isLoading: false, achievements: achievements);
+    } catch (e) {
+      _state = _state.copyWith(
+        isLoading: false,
+        errorMessage: '加载成就列表失败',
+      );
+    }
+    notifyListeners();
+  }
+}