fix(app): 修复 P2~P4 共 10 项前端问题

P2 必须修复:
- 教师布置主题 classId 从硬编码改为班级下拉选择器
- 班级日记墙使用服务端 classId 过滤替代前端过滤
- Profile 统计栏接入 JournalRepository 真实数据
- WeeklyPage 从全硬编码改为 JournalRepository 数据驱动

P3 建议改进:
- 提取 mood_utils.dart 公共函数，消除 4 处重复定义
- 贴纸库搜索框连接 StickerBloc 按名称过滤

P4 细节打磨:
- 家长页多孩子时显示 DropdownButton 选择器
- 搜索结果日记卡片点击跳转 /editor?id=
- MonthlyPage 照片数量从 JournalElement 统计
- calendar_page/mood_page/search_page 统一使用 moodToEmoji/moodToLabel

app/lib/data/repositories/isar_journal_repository.dart

@@ -34,6 +34,7 @@ class IsarJournalRepository implements JournalRepository {
     int? pageSize,
     String? mood,
     String? tag,
+    String? classId,
   }) async {
     var query = _isar.journalEntryCollections
         .where()
@@ -58,6 +59,11 @@ class IsarJournalRepository implements JournalRepository {
       query = query.and().tagsJsonContains(tag);
     }
 
+    // 班级过滤
+    if (classId != null) {
+      query = query.and().classIdEqualTo(classId);
+    }
+
     // 按日期降序排列
     var results = await query
         .sortByDateEpochDesc()
@@ -74,6 +80,15 @@ class IsarJournalRepository implements JournalRepository {
     return results.map(_fromCollection).toList();
   }
 
+  @override
+  Future<int> getJournalCount() async {
+    return _isar.journalEntryCollections
+        .where()
+        .filter()
+        .isDeletedEqualTo(false)
+        .count();
+  }
+
   @override
   Future<JournalEntry?> getJournal(String id) async {
     final col = await _isar.journalEntryCollections
@@ -262,6 +277,7 @@ class IsarJournalRepository implements JournalRepository {
       ..isPrivate = entry.isPrivate
       ..sharedToClass = entry.sharedToClass
       ..assignedTopicId = entry.assignedTopicId
+      ..contentExcerpt = entry.contentExcerpt
       ..version = entry.version
       ..createdAtEpoch = entry.createdAt.millisecondsSinceEpoch
       ..updatedAtEpoch = entry.updatedAt.millisecondsSinceEpoch
@@ -290,6 +306,7 @@ class IsarJournalRepository implements JournalRepository {
       isPrivate: col.isPrivate,
       sharedToClass: col.sharedToClass,
       assignedTopicId: col.assignedTopicId,
+      contentExcerpt: col.contentExcerpt,
       version: col.version,
       createdAt: DateTime.fromMillisecondsSinceEpoch(col.createdAtEpoch),
       updatedAt: DateTime.fromMillisecondsSinceEpoch(col.updatedAtEpoch),

app/lib/data/repositories/journal_repository.dart

@@ -15,7 +15,7 @@ import '../models/journal_element.dart';
 /// - [dateFrom]/[dateTo]: 日期范围过滤（闭区间）
 /// - [page]/[pageSize]: 分页参数，从 1 开始
 abstract class JournalRepository {
-  /// 获取日记列表（支持日期范围、心情、标签过滤和分页）
+  /// 获取日记列表（支持日期范围、心情、标签、班级过滤和分页）
   Future<List<JournalEntry>> getJournals({
     DateTime? dateFrom,
     DateTime? dateTo,
@@ -23,8 +23,12 @@ abstract class JournalRepository {
     int? pageSize,
     String? mood,
     String? tag,
+    String? classId,
   });
 
+  /// 获取日记总数
+  Future<int> getJournalCount();
+
   /// 获取单篇日记（返回 null 表示不存在）
   Future<JournalEntry?> getJournal(String id);
 
@@ -66,6 +70,7 @@ class InMemoryJournalRepository implements JournalRepository {
     int? pageSize,
     String? mood,
     String? tag,
+    String? classId,
   }) async {
     var results = _journals.values.toList();
 
@@ -87,6 +92,11 @@ class InMemoryJournalRepository implements JournalRepository {
       results = results.where((j) => j.tags.contains(tag)).toList();
     }
 
+    // 班级过滤
+    if (classId != null) {
+      results = results.where((j) => j.classId == classId).toList();
+    }
+
     // 按日期降序排列（最新在前）
     results.sort((a, b) => b.date.compareTo(a.date));
 
@@ -101,6 +111,9 @@ class InMemoryJournalRepository implements JournalRepository {
     return results;
   }
 
+  @override
+  Future<int> getJournalCount() async => _journals.length;
+
   @override
   Future<JournalEntry?> getJournal(String id) async {
     return _journals[id];

app/lib/data/repositories/remote_journal_repository.dart

@@ -21,6 +21,7 @@ class RemoteJournalRepository implements JournalRepository {
     int? pageSize,
     String? mood,
     String? tag,
+    String? classId,
   }) async {
     final queryParams = <String, dynamic>{};
     // 后端 NaiveDateTime 格式: "2026-06-01T00:00:00"（不带毫秒）
@@ -34,6 +35,7 @@ class RemoteJournalRepository implements JournalRepository {
     if (pageSize != null) queryParams['page_size'] = pageSize;
     if (mood != null) queryParams['mood'] = mood;
     if (tag != null) queryParams['tag'] = tag;
+    if (classId != null) queryParams['class_id'] = classId;
 
     final response = await _api.get('/diary/journals', queryParams: queryParams);
     final body = response.data as Map<String, dynamic>;
@@ -43,6 +45,16 @@ class RemoteJournalRepository implements JournalRepository {
         .toList();
   }
 
+  @override
+  Future<int> getJournalCount() async {
+    final response = await _api.get('/diary/journals', queryParams: {
+      'page': 1,
+      'page_size': 1,
+    });
+    final body = response.data as Map<String, dynamic>;
+    return (body['total'] as int?) ?? 0;
+  }
+
   @override
   Future<JournalEntry?> getJournal(String id) async {
     try {