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/skeleton_loading.dart

@@ -0,0 +1,156 @@
+// 共享骨架屏加载组件 — 替代 CircularProgressIndicator
+//
+// 使用: SkeletonBox(height: 80) 或 SkeletonBox(height: 48, shimmer: true)
+// 样式: 灰色圆角矩形 + 可选 shimmer 微光动画
+
+import 'package:flutter/material.dart';
+
+import '../core/theme/app_radius.dart';
+
+/// 骨架屏加载占位块
+class SkeletonBox extends StatelessWidget {
+  const SkeletonBox({
+    super.key,
+    required this.height,
+    this.width,
+    this.shimmer = false,
+    this.borderRadius,
+  });
+
+  /// 高度
+  final double height;
+
+  /// 宽度（默认撑满父容器）
+  final double? width;
+
+  /// 是否启用 shimmer 微光动画
+  final bool shimmer;
+
+  /// 自定义圆角（默认 AppRadius.md）
+  final BorderRadius? borderRadius;
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    final bgColor = theme.colorScheme.surfaceContainerHighest.withValues(alpha: 0.3);
+    final radius = borderRadius ?? AppRadius.mdBorder;
+
+    if (shimmer) {
+      return _ShimmerBox(
+        height: height,
+        width: width,
+        bgColor: bgColor,
+        borderRadius: radius,
+      );
+    }
+
+    return Container(
+      height: height,
+      width: width,
+      decoration: BoxDecoration(
+        color: bgColor,
+        borderRadius: radius,
+      ),
+    );
+  }
+}
+
+/// 带微光动画的骨架屏
+class _ShimmerBox extends StatefulWidget {
+  const _ShimmerBox({
+    required this.height,
+    this.width,
+    required this.bgColor,
+    required this.borderRadius,
+  });
+
+  final double height;
+  final double? width;
+  final Color bgColor;
+  final BorderRadius borderRadius;
+
+  @override
+  State<_ShimmerBox> createState() => _ShimmerBoxState();
+}
+
+class _ShimmerBoxState extends State<_ShimmerBox>
+    with SingleTickerProviderStateMixin {
+  late final AnimationController _controller;
+
+  @override
+  void initState() {
+    super.initState();
+    _controller = AnimationController(
+      vsync: this,
+      duration: const Duration(milliseconds: 1500),
+    )..repeat();
+  }
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    return AnimatedBuilder(
+      animation: _controller,
+      builder: (context, child) {
+        return Container(
+          height: widget.height,
+          width: widget.width,
+          decoration: BoxDecoration(
+            borderRadius: widget.borderRadius,
+            gradient: LinearGradient(
+              begin: Alignment(-1.0 + 2.0 * _controller.value, 0),
+              end: Alignment(1.0 + 2.0 * _controller.value, 0),
+              colors: [
+                widget.bgColor,
+                theme.colorScheme.surfaceContainerHighest.withValues(alpha: 0.15),
+                widget.bgColor,
+              ],
+            ),
+          ),
+        );
+      },
+    );
+  }
+}
+
+/// 通用列表骨架屏 — 显示 N 行占位
+class SkeletonList extends StatelessWidget {
+  const SkeletonList({
+    super.key,
+    this.itemCount = 5,
+    this.itemHeight = 72,
+    this.spacing = 12,
+    this.shimmer = false,
+  });
+
+  /// 骨架行数
+  final int itemCount;
+
+  /// 每行高度
+  final double itemHeight;
+
+  /// 行间距
+  final double spacing;
+
+  /// 是否 shimmer
+  final bool shimmer;
+
+  @override
+  Widget build(BuildContext context) {
+    return ListView.builder(
+      physics: const NeverScrollableScrollPhysics(),
+      shrinkWrap: true,
+      itemCount: itemCount,
+      itemBuilder: (_, _) => Padding(
+        padding: EdgeInsets.only(bottom: spacing),
+        child: SkeletonBox(height: itemHeight, shimmer: shimmer),
+      ),
+    );
+  }
+}