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

pubspec.yaml

@@ -2,7 +2,7 @@ name: new_learn
 description: "A new Flutter project."
 # The following line prevents the package from being accidentally published to
 # pub.dev using `flutter pub publish`. This is preferred for private packages.
-publish_to: 'none' # Remove this line if you wish to publish to pub.dev
+publish_to: "none" # Remove this line if you wish to publish to pub.dev
 
 # The following defines the version and build number for your application.
 # A version number is three numbers separated by dots, like 1.2.43
@@ -222,7 +222,6 @@ dev_dependencies:
 
 # The following section is specific to Flutter packages.
 flutter:
-
   # The following line ensures that the Material Icons font is
   # included with your application, so that you can use the icons in
   # the material Icons class.
@@ -236,6 +235,7 @@ flutter:
     - assets/img/tabbar/
     - assets/img/tabbar/default/
     - assets/audio/
+    - assets/img/class/
 
   # To add assets to your application, add an assets section, like this:
   # assets: