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/class_/views/class_page.dart

@@ -9,6 +9,8 @@ import 'package:nuanji_app/data/models/journal_entry.dart';
 import 'package:nuanji_app/data/models/school_class.dart';
 import 'package:nuanji_app/data/repositories/class_repository.dart';
 import 'package:nuanji_app/data/repositories/journal_repository.dart';
+import '../../../widgets/empty_state_widget.dart';
+import '../../../widgets/error_state_widget.dart';
 import '../../auth/bloc/auth_bloc.dart';
 import '../bloc/class_bloc.dart';
 import '../widgets/comment_bottom_sheet.dart';
@@ -46,7 +48,10 @@ class _ClassView extends StatelessWidget {
         if (state is ClassError) {
           return Scaffold(
             appBar: AppBar(title: const Text('班级')),
-            body: Center(child: Text(state.message)),
+            body: ErrorStateWidget(
+              message: state.message,
+              onRetry: () => context.read<ClassBloc>().add(const ClassLoadMyClasses()),
+            ),
           );
         }
 
@@ -93,22 +98,11 @@ class _ClassListView extends StatelessWidget {
   }
 
   Widget _buildEmptyState(BuildContext context, ColorScheme colorScheme) {
-    return Center(
-      child: Column(
-        mainAxisSize: MainAxisSize.min,
-        children: [
-          Icon(Icons.groups_outlined, size: 64, color: colorScheme.onSurface.withValues(alpha: 0.2)),
-          const SizedBox(height: 16),
-          Text('还没有加入任何班级', style: Theme.of(context).textTheme.bodyLarge?.copyWith(
-            color: colorScheme.onSurface.withValues(alpha: 0.5),
-          )),
-          const SizedBox(height: 24),
-          FilledButton.tonal(
-            onPressed: () => context.go('/class-code'),
-            child: const Text('输入班级码加入'),
-          ),
-        ],
-      ),
+    return EmptyStateWidget(
+      icon: Icons.group_add_rounded,
+      title: '还没有加入班级',
+      actionLabel: '通过班级码加入',
+      onAction: () => context.go('/class-code'),
     );
   }
 }
@@ -250,21 +244,11 @@ class _DiaryWallTab extends StatelessWidget {
     }
 
     if (state.diaryWall.isEmpty) {
-      return Center(
-        child: Column(
-          mainAxisSize: MainAxisSize.min,
-          children: [
-            Icon(Icons.auto_stories_outlined, size: 48, color: colorScheme.onSurface.withValues(alpha: 0.2)),
-            const SizedBox(height: 12),
-            Text('日记墙还是空的', style: theme.textTheme.bodyLarge?.copyWith(
-              color: colorScheme.onSurface.withValues(alpha: 0.5),
-            )),
-            const SizedBox(height: 8),
-            Text('分享你的日记到班级吧！', style: theme.textTheme.bodySmall?.copyWith(
-              color: colorScheme.onSurface.withValues(alpha: 0.3),
-            )),
-          ],
-        ),
+      return const EmptyStateWidget(
+        icon: Icons.auto_stories_rounded,
+        title: '日记墙还是空的',
+        subtitle: '分享你的日记到这里吧',
+        iconSize: 48,
       );
     }
 
@@ -441,10 +425,9 @@ class _TopicsTab extends StatelessWidget {
     final colorScheme = theme.colorScheme;
 
     if (topics.isEmpty) {
-      return Center(
-        child: Text('暂无主题布置', style: theme.textTheme.bodyLarge?.copyWith(
-          color: colorScheme.onSurface.withValues(alpha: 0.5),
-        )),
+      return const EmptyStateWidget(
+        icon: Icons.assignment_outlined,
+        title: '暂无主题布置',
       );
     }