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/class_/views/class_page.dart

@@ -344,31 +344,35 @@ class _DiaryWallCard extends StatelessWidget {
                   color: colorScheme.onSurface.withValues(alpha: 0.4),
                 ),
               ),
-              // 评语
-              if (comments.isNotEmpty) ...[
-                const SizedBox(height: 8),
-                Container(
-                  padding: const EdgeInsets.all(8),
-                  decoration: BoxDecoration(
-                    color: colorScheme.primaryContainer.withValues(alpha: 0.3),
-                    borderRadius: AppRadius.smBorder,
-                  ),
-                  child: Row(
-                    children: [
-                      const Icon(Icons.rate_review_rounded, size: 14),
-                      const SizedBox(width: 4),
-                      Expanded(
-                        child: Text(
-                          comments.first.content,
-                          style: theme.textTheme.bodySmall,
-                          maxLines: 2,
-                          overflow: TextOverflow.ellipsis,
+              // 评语（按 journalId 过滤，避免显示在错误卡片下）
+              ...(() {
+                final journalComments = comments.where((c) => c.journalId == journal.id).toList();
+                if (journalComments.isEmpty) return <Widget>[];
+                return [
+                  const SizedBox(height: 8),
+                  Container(
+                    padding: const EdgeInsets.all(8),
+                    decoration: BoxDecoration(
+                      color: colorScheme.primaryContainer.withValues(alpha: 0.3),
+                      borderRadius: AppRadius.smBorder,
+                    ),
+                    child: Row(
+                      children: [
+                        const Icon(Icons.rate_review_rounded, size: 14),
+                        const SizedBox(width: 4),
+                        Expanded(
+                          child: Text(
+                            journalComments.first.content,
+                            style: theme.textTheme.bodySmall,
+                            maxLines: 2,
+                            overflow: TextOverflow.ellipsis,
+                          ),
                         ),
-                      ),
-                    ],
+                      ],
+                    ),
                   ),
-                ),
-              ],
+                ];
+              })(),
               // 写评语按钮（仅老师可见）
               if (_isTeacher(context)) ...[
                 const SizedBox(height: 8),