refactor(class): 抽离攻克任务组件并优化样式与资源

1.  将ResolveProblemBox从class_update_widgets.dart抽离到公共组件目录
2.  替换静态渐变背景为CDN资源背景图，适配平板和手机端
3.  将点赞评论按钮的图标从内置Icons替换为本地png资源
4.  更新pubspec.yaml添加新的图片资源路径
5.  删除冗余的空行配置

lib/features/class/widgets/class_update_widgets.dart

@@ -5,6 +5,7 @@ import 'package:flutter/material.dart';
 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/user_circle_avatar.dart';
 import 'package:new_learn/core/utils/img_url.dart';
 
@@ -12,7 +13,8 @@ import 'package:new_learn/core/utils/img_url.dart';
 /// 【班级动态组件】由 class_update_view.dart 抽离的展示组件
 ///
 /// PostCard（动态卡片，按 type 分发内容 + 相册 + 点赞评论栏）
-/// CardHeader / SubjectTag / ResolveProblemBox / AlbumDesc / AlbumGrid
+/// CardHeader / SubjectTag / AlbumDesc / AlbumGrid / LikeCommentBar / ...
+/// ResolveProblemBox 已抽离到 core/components/resolve_problem_box.dart
 /// LikeCommentBar / MetaButton / CommentPreview / CommentInput
 ///
 /// 原为 class_update_view.dart 内的私有组件，重命名去下划线后放到本文件。
@@ -263,65 +265,6 @@ class SubjectTag extends StatelessWidget {
   }
 }
 
-/// 攻克任务卡（对标 ResolveProblem）
-class ResolveProblemBox extends StatelessWidget {
-  const ResolveProblemBox({super.key, required this.desc});
-
-  final String desc;
-
-  @override
-  Widget build(BuildContext context) {
-    return Container(
-      width: double.infinity,
-      padding: EdgeInsets.fromLTRB(19.w, 25.h, 19.w, 11.h),
-      decoration: BoxDecoration(
-        gradient: const LinearGradient(
-          begin: Alignment.topCenter,
-          end: Alignment.bottomCenter,
-          colors: [Color(0xFFEAF4FF), Color(0xFFFFFFFF)],
-        ),
-        borderRadius: BorderRadius.circular(10.r),
-      ),
-      child: Column(
-        crossAxisAlignment: CrossAxisAlignment.start,
-        children: [
-          const Text(
-            '刚刚攻克了',
-            style: TextStyle(
-              fontSize: 15,
-              fontWeight: FontWeight.w900,
-              color: Color(0xFF1D2B47),
-            ),
-          ),
-          SizedBox(height: 3.h),
-          Text.rich(
-            TextSpan(
-              children: [
-                TextSpan(
-                  text: '｢$desc｣',
-                  style: const TextStyle(
-                    fontSize: 13,
-                    fontWeight: FontWeight.w500,
-                    color: Color(0xFF1D2B47),
-                  ),
-                ),
-                const TextSpan(
-                  text: ' 的所有高难考点！',
-                  style: TextStyle(
-                    fontSize: 13,
-                    fontWeight: FontWeight.w500,
-                    color: Color(0xFF45597E),
-                  ),
-                ),
-              ],
-            ),
-          ),
-        ],
-      ),
-    );
-  }
-}
-
 /// 纯文字笔记（album-desc，最多 2 行）
 class AlbumDesc extends StatelessWidget {
   const AlbumDesc({super.key, required this.text});
@@ -441,15 +384,26 @@ class LikeCommentBar extends StatelessWidget {
           mainAxisAlignment: MainAxisAlignment.end,
           children: [
             MetaButton(
-              icon: post.userLikeStatus
-                  ? Icons.favorite
-                  : Icons.favorite_border,
+              icon: Image.asset(
+                post.userLikeStatus
+                    ? 'assets/img/class/like-check-icon.png'
+                    : 'assets/img/class/like-icon.png',
+                width: 13.w,
+                height: 13.w,
+              ),
               count: post.likeCount,
               active: post.userLikeStatus,
               onTap: onLike,
             ),
             SizedBox(width: 8.w),
-            MetaButton(icon: Icons.chat_bubble_outline, count: post.replyCount),
+            MetaButton(
+              icon: Image.asset(
+                'assets/img/class/comment-icon.png',
+                width: 13.w,
+                height: 13.w,
+              ),
+              count: post.replyCount,
+            ),
           ],
         ),
         if (showComposer) ...[
@@ -474,7 +428,7 @@ class MetaButton extends StatelessWidget {
     this.onTap,
   });
 
-  final IconData icon;
+  final Widget icon;
   final int count;
   final bool active;
   final VoidCallback? onTap;
@@ -496,7 +450,7 @@ class MetaButton extends StatelessWidget {
         child: Row(
           mainAxisAlignment: MainAxisAlignment.center,
           children: [
-            Icon(icon, size: 13.w, color: color),
+            icon,
             SizedBox(width: 4.w),
             Text('$count', style: TextStyle(fontSize: 13, color: color)),
           ],