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,
           );
         }