refactor(app): 迁移 4 个页面到共享 EmptyStateWidget + ErrorStateWidget

迁移统计:
- discover_page: _buildError → ErrorStateWidget, _buildEmptyHint → EmptyStateWidget
- sticker_library_page: 错误 + 空列表 → 共享组件
- class_page: 错误/班级列表空/日记墙空/话题空 → 共享组件 (4 处)
- calendar_page: CalendarError → ErrorStateWidget

统一体验: 所有页面空状态使用一致的 icon + title + subtitle + CTA 布局

app/lib/features/discover/views/discover_page.dart

@@ -19,6 +19,8 @@ import '../../../core/theme/app_colors.dart';
 import '../../../core/theme/app_radius.dart';
 import '../../../core/theme/app_shadows.dart';
 import '../../../core/theme/app_typography.dart';
+import '../../../widgets/empty_state_widget.dart';
+import '../../../widgets/error_state_widget.dart';
 import '../bloc/discover_bloc.dart';
 import '../models/discover_models.dart';
 
@@ -126,66 +128,19 @@ class DiscoverPage extends StatelessWidget {
 
   /// 错误状态
   Widget _buildError(BuildContext context, String message) {
-    return Column(
-      children: [
-        const _LoadingSkeleton(height: 140),
-        const SizedBox(height: DesignTokens.spacing24),
-        Container(
-          width: double.infinity,
-          padding: const EdgeInsets.all(DesignTokens.spacing16),
-          decoration: BoxDecoration(
-            color: AppColors.rose.withValues(alpha: 0.1),
-            borderRadius: AppRadius.mdBorder,
-          ),
-          child: Column(
-            children: [
-              Icon(Icons.cloud_off_rounded,
-                  size: 32, color: AppColors.rose),
-              const SizedBox(height: DesignTokens.spacing8),
-              Text(message,
-                  style: TextStyle(
-                      fontSize: 13,
-                      color: Theme.of(context).colorScheme.onSurfaceVariant),
-                  textAlign: TextAlign.center),
-              const SizedBox(height: DesignTokens.spacing12),
-              TextButton.icon(
-                onPressed: () => context
-                    .read<DiscoverBloc>()
-                    .add(const DiscoverLoadData()),
-                icon: const Icon(Icons.refresh_rounded, size: 18),
-                label: const Text('重试'),
-              ),
-            ],
-          ),
-        ),
-      ],
+    return ErrorStateWidget(
+      message: message,
+      onRetry: () =>
+          context.read<DiscoverBloc>().add(const DiscoverLoadData()),
     );
   }
 
   /// 空数据提示
   Widget _buildEmptyHint(BuildContext context) {
-    return Padding(
-      padding: const EdgeInsets.symmetric(vertical: DesignTokens.spacing32),
-      child: Center(
-        child: Column(
-          children: [
-            const Text('✨', style: TextStyle(fontSize: 40)),
-            const SizedBox(height: DesignTokens.spacing12),
-            Text('还没有发现内容',
-                style: TextStyle(
-                  fontSize: 15,
-                  color: Theme.of(context).colorScheme.onSurfaceVariant,
-                )),
-            const SizedBox(height: 4),
-            Text('写下你的第一篇日记，出现在这里吧！',
-                style: TextStyle(
-                  fontSize: 13,
-                  color: Theme.of(context).colorScheme.onSurfaceVariant
-                      .withValues(alpha: 0.7),
-                )),
-          ],
-        ),
-      ),
+    return const EmptyStateWidget(
+      icon: Icons.explore_rounded,
+      title: '还没有发现内容',
+      subtitle: '试试写一篇日记分享给大家吧',
     );
   }