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/calendar/views/calendar_page.dart

@@ -9,6 +9,7 @@ import 'package:nuanji_app/core/theme/app_radius.dart';
 import 'package:nuanji_app/core/utils/mood_utils.dart';
 import 'package:nuanji_app/data/models/journal_entry.dart';
 import 'package:nuanji_app/data/repositories/journal_repository.dart';
+import '../../../widgets/error_state_widget.dart';
 import '../bloc/calendar_bloc.dart';
 
 /// 日历页面 — 月视图(心情色彩) + 周视图 + 时间轴
@@ -41,21 +42,11 @@ class _CalendarView extends StatelessWidget {
         }
 
         if (state is CalendarError) {
-          return Center(
-            child: Column(
-              mainAxisSize: MainAxisSize.min,
-              children: [
-                Icon(Icons.error_outline, size: 48, color: colorScheme.error),
-                const SizedBox(height: 16),
-                Text(state.message, style: theme.textTheme.bodyLarge),
-                const SizedBox(height: 16),
-                FilledButton.tonal(
-                  onPressed: () => context.read<CalendarBloc>()
-                      .add(CalendarMonthChanged(DateTime.now())),
-                  child: const Text('重试'),
-                ),
-              ],
-            ),
+          return ErrorStateWidget(
+            message: state.message,
+            onRetry: () => context.read<CalendarBloc>()
+                .add(CalendarMonthChanged(DateTime.now())),
+            icon: Icons.error_outline,
           );
         }
 

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',
       );
     }
 

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',
     );
   }
 

app/lib/features/stickers/views/sticker_library_page.dart

@@ -5,6 +5,8 @@ import 'package:flutter_bloc/flutter_bloc.dart';
 import 'package:nuanji_app/core/theme/app_colors.dart';
 import 'package:nuanji_app/core/theme/app_radius.dart';
 import 'package:nuanji_app/data/remote/api_client.dart';
+import '../../../widgets/empty_state_widget.dart';
+import '../../../widgets/error_state_widget.dart';
 import '../bloc/sticker_bloc.dart';
 
 /// 贴纸库页面 — 分类浏览贴纸包
@@ -64,18 +66,10 @@ class _StickerLibraryPageState extends State<StickerLibraryPage> {
             }
 
             if (state.errorMessage != null) {
-              return Center(
-                child: Column(
-                  mainAxisAlignment: MainAxisAlignment.center,
-                  children: [
-                    Icon(Icons.error_outline, size: 48, color: colorScheme.error),
-                    const SizedBox(height: 16),
-                    FilledButton.tonal(
-                      onPressed: _bloc.load,
-                      child: const Text('重试'),
-                    ),
-                  ],
-                ),
+              return ErrorStateWidget(
+                message: state.errorMessage ?? '加载失败',
+                onRetry: _bloc.load,
+                icon: Icons.error_outline,
               );
             }
 
@@ -169,7 +163,10 @@ class _StickerLibraryPageState extends State<StickerLibraryPage> {
                 // ---- 贴纸包网格 ----
                 Expanded(
                   child: state.filteredPacks.isEmpty
-                      ? const Center(child: Text('暂无贴纸包'))
+                      ? const EmptyStateWidget(
+                          icon: Icons.sticky_note_2_outlined,
+                          title: '暂无贴纸包',
+                        )
                       : GridView.builder(
                           padding: const EdgeInsets.all(16),
                           gridDelegate: