feat(my): 新增错题本和我的收藏页面，重构学科标签组件

1.  为我的页面添加错题本和我的收藏跳转路由，指向共用的QuestionListPage
2.  抽离全局学科标签组件，移除class模块内的重复实现
3.  新增用户收藏/错题本接口层和数据模型
4.  完成QuestionListPage分页列表页实现，支持学科筛选和游标分页
5.  调整考试页面卡片布局和图标偏移参数

lib/features/class/widgets/class_update_widgets.dart

@@ -6,6 +6,7 @@ import 'package:flutter_screenutil/flutter_screenutil.dart';
 import 'package:new_learn/core/api/class/model.dart';
 import 'package:new_learn/core/components/image_preview_viewer.dart';
 import 'package:new_learn/core/components/resolve_problem_box.dart';
+import 'package:new_learn/core/components/subject_tag.dart';
 import 'package:new_learn/core/components/user_circle_avatar.dart';
 import 'package:new_learn/core/utils/img_url.dart';
 
@@ -236,35 +237,6 @@ class CardHeader extends StatelessWidget {
   }
 }
 
-/// 学科标签（对标 SubjectTag）
-class SubjectTag extends StatelessWidget {
-  const SubjectTag({super.key, required this.subject});
-
-  final String subject;
-
-  static const Map<String, _SubjectColor> _map = {
-    '数学': _SubjectColor(0xFF3D7DF5, 0xFFE5F0FF),
-    '物理': _SubjectColor(0xFFA45CFF, 0xFFF5ECFF),
-    '英语': _SubjectColor(0xFFE5A301, 0xFFFFF6E3),
-    '化学': _SubjectColor(0xFF2DB985, 0xFFE5FAF3),
-  };
-
-  @override
-  Widget build(BuildContext context) {
-    final t = subject.trim();
-    if (t.isEmpty) return const SizedBox.shrink();
-    final c = _map[t] ?? const _SubjectColor(0xFF3D7DF5, 0xFFE5F0FF);
-    return Container(
-      padding: EdgeInsets.symmetric(horizontal: 7.w, vertical: 2.h),
-      decoration: BoxDecoration(
-        color: Color(c.bg),
-        borderRadius: BorderRadius.circular(6.r),
-      ),
-      child: Text(t, style: TextStyle(fontSize: 11, color: Color(c.fg))),
-    );
-  }
-}
-
 /// 纯文字笔记（album-desc，最多 2 行）
 class AlbumDesc extends StatelessWidget {
   const AlbumDesc({super.key, required this.text});
@@ -613,9 +585,3 @@ class _CommentInputState extends State<CommentInput> {
     );
   }
 }
-
-class _SubjectColor {
-  const _SubjectColor(this.fg, this.bg);
-  final int fg;
-  final int bg;
-}