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/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: