feat(app): BLoC 集成 Repository + SettingsBloc 主题切换

全局依赖注入:
- app.dart 注入 JournalRepository + ClassRepository + SettingsBloc
- ApiClient token 自动注入（监听 AuthBloc 状态）

BLoC 重构 (占位数据 → Repository):
- CalendarBloc: 通过 JournalRepository 加载月度日记
- ClassBloc: 通过 ClassRepository + JournalRepository 加载班级数据
- 新增 ClassJoin 事件支持班级码加入
- HomeBloc: 加载最近日记 + 心情概览 + 连续天数 + 今日是否已写

设置系统:
- SettingsBloc: ThemeMode 切换 (system/light/dark)
- app.dart 通过 ListenableBuilder 响应主题变化
- HomeBloc 支持下拉刷新

首页增强:
- 连续天数徽章 + 今日已写标记 + 最常用心情高亮
- RefreshIndicator 下拉刷新
- 日记列表卡片显示日期

验证: flutter analyze 0 error

app/lib/features/home/bloc/home_bloc.dart

@@ -0,0 +1,139 @@
+// 首页 BLoC — 加载最近日记和心情概览
+
+import 'package:flutter_bloc/flutter_bloc.dart';
+import 'package:nuanji_app/data/models/journal_entry.dart';
+import 'package:nuanji_app/data/repositories/journal_repository.dart';
+
+// ===== Events =====
+
+sealed class HomeEvent {
+  const HomeEvent();
+}
+
+/// 加载首页数据（最近日记 + 心情概览）
+final class HomeLoadData extends HomeEvent {
+  const HomeLoadData();
+}
+
+/// 刷新首页
+final class HomeRefresh extends HomeEvent {
+  const HomeRefresh();
+}
+
+// ===== State =====
+
+/// 首页状态
+sealed class HomeState {
+  const HomeState();
+}
+
+final class HomeInitial extends HomeState {
+  const HomeInitial();
+}
+
+final class HomeLoading extends HomeState {
+  const HomeLoading();
+}
+
+final class HomeLoaded extends HomeState {
+  /// 最近日记（取最新 10 条）
+  final List<JournalEntry> recentJournals;
+
+  /// 今日是否已写日记
+  final bool hasTodayEntry;
+
+  /// 最近常用心情
+  final Mood? topMood;
+
+  /// 连续写日记天数（从日记列表推算）
+  final int streakDays;
+
+  const HomeLoaded({
+    this.recentJournals = const [],
+    this.hasTodayEntry = false,
+    this.topMood,
+    this.streakDays = 0,
+  });
+}
+
+final class HomeError extends HomeState {
+  final String message;
+  const HomeError(this.message);
+}
+
+// ===== BLoC =====
+
+class HomeBloc extends Bloc<HomeEvent, HomeState> {
+  final JournalRepository _journalRepo;
+
+  HomeBloc({required JournalRepository journalRepository})
+      : _journalRepo = journalRepository,
+        super(const HomeInitial()) {
+    on<HomeLoadData>(_onLoadData);
+    on<HomeRefresh>(_onRefresh);
+  }
+
+  Future<void> _onLoadData(
+    HomeLoadData event,
+    Emitter<HomeState> emit,
+  ) async {
+    emit(const HomeLoading());
+    try {
+      final journals = await _journalRepo.getJournals(
+        page: 1,
+        pageSize: 10,
+      );
+
+      // 检查今日是否已写日记
+      final today = DateTime.now();
+      final hasTodayEntry = journals.any((j) =>
+          j.date.year == today.year &&
+          j.date.month == today.month &&
+          j.date.day == today.day);
+
+      // 推算最常用心情
+      final moodCounts = <Mood, int>{};
+      for (final j in journals) {
+        moodCounts[j.mood] = (moodCounts[j.mood] ?? 0) + 1;
+      }
+      final topMood = moodCounts.entries
+          .fold<MapEntry<Mood, int>?>(null, (a, b) => a == null || b.value > a.value ? b : a)
+          ?.key;
+
+      // 推算连续天数
+      final streakDays = _calculateStreak(journals);
+
+      emit(HomeLoaded(
+        recentJournals: journals,
+        hasTodayEntry: hasTodayEntry,
+        topMood: topMood,
+        streakDays: streakDays,
+      ));
+    } catch (e) {
+      emit(const HomeLoaded()); // 空状态而非错误，离线友好
+    }
+  }
+
+  Future<void> _onRefresh(
+    HomeRefresh event,
+    Emitter<HomeState> emit,
+  ) async {
+    add(const HomeLoadData());
+  }
+
+  /// 从日记列表推算连续写日记天数
+  int _calculateStreak(List<JournalEntry> journals) {
+    if (journals.isEmpty) return 0;
+
+    final dates = journals.map((j) => j.date).toSet();
+    var streak = 0;
+    var checkDate = DateTime.now();
+
+    while (dates.contains(DateTime(checkDate.year, checkDate.month, checkDate.day))) {
+      streak++;
+      checkDate = checkDate.subtract(const Duration(days: 1));
+    }
+
+    return streak;
+  }
+}