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/error_state_widget.dart

@@ -0,0 +1,72 @@
+// 共享错误状态组件 — 统一所有页面的错误展示
+//
+// 使用: ErrorStateWidget(message: '加载失败', onRetry: () => ...)
+// 样式: 云朵图标 + 错误信息 + 重试按钮
+
+import 'package:flutter/material.dart';
+
+import '../core/constants/design_tokens.dart';
+import '../core/theme/app_radius.dart';
+
+/// 统一错误状态组件 — 图标 + 错误信息 + 可选重试按钮
+class ErrorStateWidget extends StatelessWidget {
+  const ErrorStateWidget({
+    super.key,
+    required this.message,
+    this.onRetry,
+    this.icon = Icons.cloud_off_rounded,
+  });
+
+  /// 错误描述文字
+  final String message;
+
+  /// 重试回调（不传则不显示重试按钮）
+  final VoidCallback? onRetry;
+
+  /// 主图标（默认云朵离线图标）
+  final IconData icon;
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    return Center(
+      child: Padding(
+        padding: const EdgeInsets.symmetric(
+          horizontal: DesignTokens.spacing24,
+          vertical: DesignTokens.spacing40,
+        ),
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            Icon(
+              icon,
+              size: 56,
+              color: theme.colorScheme.onSurface.withValues(alpha: 0.25),
+            ),
+            const SizedBox(height: DesignTokens.spacing16),
+            Text(
+              message,
+              style: theme.textTheme.bodyMedium?.copyWith(
+                color: theme.colorScheme.onSurfaceVariant,
+              ),
+              textAlign: TextAlign.center,
+            ),
+            if (onRetry != null) ...[
+              const SizedBox(height: DesignTokens.spacing20),
+              TextButton.icon(
+                onPressed: onRetry,
+                icon: const Icon(Icons.refresh_rounded, size: 18),
+                label: const Text('重试'),
+                style: TextButton.styleFrom(
+                  shape: RoundedRectangleBorder(
+                    borderRadius: AppRadius.pillBorder,
+                  ),
+                ),
+              ),
+            ],
+          ],
+        ),
+      ),
+    );
+  }
+}