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/features/parent/views/parent_page.dart

@@ -535,7 +535,7 @@ class _ChildCard extends StatelessWidget {
 }
 
 /// 功能操作网格 — 4 个功能按钮
-class _ActionGrid extends StatelessWidget {
+class _ActionGrid extends StatefulWidget {
   const _ActionGrid({
     required this.children,
     required this.onViewJournals,
@@ -550,20 +550,88 @@ class _ActionGrid extends StatelessWidget {
   final void Function(String childId) onDelete;
   final void Function(String childId) onMoodStats;
 
-  /// 取第一个绑定的孩子 ID（Phase 1 简化逻辑）
-  String get _firstChildId =>
-      children.isNotEmpty ? children.first.childId : '';
+  @override
+  State<_ActionGrid> createState() => _ActionGridState();
+}
+
+class _ActionGridState extends State<_ActionGrid> {
+  late String _selectedChildId;
+
+  @override
+  void initState() {
+    super.initState();
+    _selectedChildId = widget.children.isNotEmpty ? widget.children.first.childId : '';
+  }
+
+  @override
+  void didUpdateWidget(covariant _ActionGrid oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    // 当孩子列表变化时更新选中 ID
+    if (oldWidget.children != widget.children) {
+      if (!widget.children.any((c) => c.childId == _selectedChildId)) {
+        _selectedChildId = widget.children.isNotEmpty ? widget.children.first.childId : '';
+      }
+    }
+  }
 
   @override
   Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    final colorScheme = theme.colorScheme;
+
     return Column(
       crossAxisAlignment: CrossAxisAlignment.start,
       children: [
+        // 孩子选择器（多孩子时显示）
+        if (widget.children.length > 1) ...[
+          Text(
+            '选择孩子',
+            style: theme.textTheme.titleSmall?.copyWith(
+              fontWeight: FontWeight.w600,
+            ),
+          ),
+          const SizedBox(height: 8),
+          Container(
+            padding: const EdgeInsets.symmetric(horizontal: 12),
+            decoration: BoxDecoration(
+              color: colorScheme.surface,
+              borderRadius: AppRadius.mdBorder,
+              border: Border.all(color: colorScheme.outlineVariant),
+            ),
+            child: DropdownButtonHideUnderline(
+              child: DropdownButton<String>(
+                value: _selectedChildId.isEmpty ? null : _selectedChildId,
+                isExpanded: true,
+                icon: const Icon(Icons.expand_more, size: 20),
+                items: widget.children.map((child) {
+                  final shortId = child.childId.length > 8
+                      ? child.childId.substring(0, 8)
+                      : child.childId;
+                  return DropdownMenuItem(
+                    value: child.childId,
+                    child: Row(
+                      children: [
+                        const Text('👧', style: TextStyle(fontSize: 18)),
+                        const SizedBox(width: 8),
+                        Text('孩子 $shortId'),
+                      ],
+                    ),
+                  );
+                }).toList(),
+                onChanged: (v) {
+                  if (v != null) setState(() => _selectedChildId = v);
+                },
+              ),
+            ),
+          ),
+          const SizedBox(height: 16),
+        ],
+
         Text(
           '功能',
-          style: Theme.of(context).textTheme.titleSmall?.copyWith(
-                fontWeight: FontWeight.w600,
-              ),
+          style: theme.textTheme.titleSmall?.copyWith(
+            fontWeight: FontWeight.w600,
+          ),
         ),
         const SizedBox(height: 12),
         _ActionCard(
@@ -572,7 +640,7 @@ class _ActionGrid extends StatelessWidget {
           iconBgColor: AppColors.accent.withValues(alpha: 0.12),
           title: '日记查看',
           subtitle: '只读查看孩子的日记和评语',
-          onTap: () => onViewJournals(_firstChildId),
+          onTap: () => widget.onViewJournals(_selectedChildId),
         ),
         const SizedBox(height: 12),
         _ActionCard(
@@ -581,7 +649,7 @@ class _ActionGrid extends StatelessWidget {
           iconBgColor: AppColors.secondary.withValues(alpha: 0.12),
           title: '心情统计',
           subtitle: '查看孩子的写作频率和心情趋势',
-          onTap: () => onMoodStats(_firstChildId),
+          onTap: () => widget.onMoodStats(_selectedChildId),
         ),
         const SizedBox(height: 12),
         _ActionCard(
@@ -590,7 +658,7 @@ class _ActionGrid extends StatelessWidget {
           iconBgColor: AppColors.tertiary.withValues(alpha: 0.12),
           title: '数据导出',
           subtitle: '导出孩子的所有日记数据',
-          onTap: () => onExport(_firstChildId),
+          onTap: () => widget.onExport(_selectedChildId),
         ),
         const SizedBox(height: 12),
         _ActionCard(
@@ -599,7 +667,7 @@ class _ActionGrid extends StatelessWidget {
           iconBgColor: AppColors.error.withValues(alpha: 0.12),
           title: '数据删除',
           subtitle: '永久删除孩子的日记数据',
-          onTap: () => onDelete(_firstChildId),
+          onTap: () => widget.onDelete(_selectedChildId),
         ),
       ],
     );