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/widgets/empty_state_widget.dart

@@ -0,0 +1,95 @@
+// 共享空状态组件 — 统一所有页面的空数据展示
+//
+// 使用: EmptyStateWidget(icon: Icons.xxx, title: '暂无数据', subtitle: '下拉刷新试试')
+// 样式参考: home_page._EmptyJournalState — 大图标淡化 + 标题 + 副标题 + 暖色按钮
+
+import 'package:flutter/material.dart';
+
+import '../core/constants/design_tokens.dart';
+import '../core/theme/app_colors.dart';
+import '../core/theme/app_radius.dart';
+
+/// 统一空状态组件 — 图标 + 标题 + 可选副标题 + 可选操作按钮
+class EmptyStateWidget extends StatelessWidget {
+  const EmptyStateWidget({
+    super.key,
+    required this.icon,
+    required this.title,
+    this.subtitle,
+    this.actionLabel,
+    this.onAction,
+    this.iconSize = 64,
+  });
+
+  /// 主图标
+  final IconData icon;
+
+  /// 图标大小
+  final double iconSize;
+
+  /// 标题文字
+  final String title;
+
+  /// 副标题（灰色小字）
+  final String? subtitle;
+
+  /// 操作按钮文字（不传则不显示按钮）
+  final String? actionLabel;
+
+  /// 操作按钮回调
+  final VoidCallback? onAction;
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    return Center(
+      child: Padding(
+        padding: const EdgeInsets.symmetric(vertical: DesignTokens.spacing40),
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            Icon(
+              icon,
+              size: iconSize,
+              color: theme.colorScheme.onSurface.withValues(alpha: 0.2),
+            ),
+            const SizedBox(height: DesignTokens.spacing16),
+            Text(
+              title,
+              style: theme.textTheme.bodyLarge?.copyWith(
+                color: theme.colorScheme.onSurfaceVariant,
+              ),
+              textAlign: TextAlign.center,
+            ),
+            if (subtitle != null) ...[
+              const SizedBox(height: DesignTokens.spacing8),
+              Text(
+                subtitle!,
+                style: theme.textTheme.bodyMedium?.copyWith(
+                  color: theme.colorScheme.onSurfaceVariant.withValues(alpha: 0.6),
+                ),
+                textAlign: TextAlign.center,
+              ),
+            ],
+            if (actionLabel != null && onAction != null) ...[
+              const SizedBox(height: DesignTokens.spacing24),
+              FilledButton.icon(
+                onPressed: onAction,
+                icon: const Icon(Icons.add_rounded),
+                label: Text(actionLabel!),
+                style: FilledButton.styleFrom(
+                  backgroundColor: AppColors.accent,
+                  foregroundColor: AppColors.bgLight,
+                  shape: RoundedRectangleBorder(
+                    borderRadius: AppRadius.pillBorder,
+                  ),
+                  padding: const EdgeInsets.symmetric(horizontal: 24, vertical: 12),
+                ),
+              ),
+            ],
+          ],
+        ),
+      ),
+    );
+  }
+}