feat(app): 共享 UI 组件 + 4 个关键 UX bug 修复

Phase 0 — 共享组件:
- EmptyStateWidget: 统一空状态 (icon + title + subtitle + CTA)
- ErrorStateWidget: 统一错误状态 (message + retry)
- SkeletonBox + SkeletonList: 统一骨架屏加载 (shimmer 动画)

Phase 1 — Bug 修复:
- 班级评论按 journalId 过滤，避免显示在错误日记卡片下
- moodCellColors key 修正: love/tired → angry/thinking
- 日历非 CalendarLoaded 状态改为加载指示器 (不再 SizedBox.shrink)
- 贴纸数统计改为 '--' 占位 (之前错误显示日记总数)

app/lib/features/calendar/views/calendar_page.dart

@@ -59,7 +59,9 @@ class _CalendarView extends StatelessWidget {
           );
         }
 
-        if (state is! CalendarLoaded) return const SizedBox.shrink();
+        if (state is! CalendarLoaded) {
+          return const Center(child: CircularProgressIndicator());
+        }
         final loaded = state;
 
         return Column(