refactor: 重构页面路由结构与积分页面UI，优化列表交互体验

1. 重构路由结构：将班级子页移出ShellBranch，解决tabbar遮挡问题
2. 统一修改全局"没有更多了"提示文案为带品牌标识的版本
3. 重构积分排行榜UI，修复切换状态时的布局抖动问题，替换图标为CDN资源
4. 重构我的积分页面：对齐Vue端UI设计，新增展开收起详情功能，优化背景与布局
5. 优化我的互动页面布局与样式，新增背景图支持

lib/features/class/my_interact_page.dart

@@ -1,8 +1,10 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_screenutil/flutter_screenutil.dart';
+import 'package:cached_network_image/cached_network_image.dart';
 import 'package:go_router/go_router.dart';
 import 'package:new_learn/core/api/class/api.dart';
 import 'package:new_learn/core/api/class/model.dart';
+import 'package:new_learn/core/utils/img_url.dart';
 import 'package:new_learn/features/class/widgets/post_list_body.dart';
 
 /// ============================================================
@@ -45,9 +47,10 @@ class _MyInteractPageState extends State<MyInteractPage> {
   Widget build(BuildContext context) {
     return Scaffold(
       backgroundColor: const Color(0xFFF5F8FA),
+      extendBodyBehindAppBar: true,
       appBar: AppBar(
-        backgroundColor: const Color(0xFFF5F8FA),
-        surfaceTintColor: Colors.transparent, // 滚动时不叠加半透明色，背景保持不变
+        backgroundColor: Colors.transparent,
+        surfaceTintColor: Colors.transparent,
         elevation: 0,
         title: const Text(
           '我的互动',
@@ -64,16 +67,45 @@ class _MyInteractPageState extends State<MyInteractPage> {
               context.canPop() ? context.pop() : context.go('/class'),
         ),
       ),
-      body: Column(
+      // 对齐 Vue .activity-box：背景图 + #F5F8FA 底色
+      // background-image: url(/StaticResource/2026-04-07/myInteract-bg.png)
+      // background-size: 100% 540rpx → height: 270.h + BoxFit.fitWidth
+      body: Stack(
         children: [
-          _buildTabs(),
-          Expanded(
-            child: IndexedStack(
-              index: _currentTab,
+          Positioned(
+            top: 0,
+            left: 0,
+            right: 0,
+            child: SizedBox(
+              height: 270.h, // Vue 540rpx
+              child: CachedNetworkImage(
+                imageUrl: ImgUrl.build(
+                  '/StaticResource/2026-04-07/myInteract-bg.png',
+                ),
+                fit: BoxFit.fitWidth,
+                placeholder: (_, __) => const SizedBox.shrink(),
+                errorWidget: (_, __, ___) => const SizedBox.shrink(),
+              ),
+            ),
+          ),
+          // 内容区：顶 padding = 系统状态栏 + AppBar 高度，对齐 Vue NavBar 后的起点
+          Padding(
+            padding: EdgeInsets.only(
+              top: MediaQuery.of(context).padding.top + kToolbarHeight,
+            ),
+            child: Column(
               children: [
-                if (_loadedTabs[0]) _UpdateTab(onTapPost: _openDetail),
-                if (_loadedTabs[1]) _LikeTab(onTapPost: _openDetail),
-                if (_loadedTabs[2]) _CommentTab(onTapPost: _openDetail),
+                _buildTabs(),
+                Expanded(
+                  child: IndexedStack(
+                    index: _currentTab,
+                    children: [
+                      if (_loadedTabs[0]) _UpdateTab(onTapPost: _openDetail),
+                      if (_loadedTabs[1]) _LikeTab(onTapPost: _openDetail),
+                      if (_loadedTabs[2]) _CommentTab(onTapPost: _openDetail),
+                    ],
+                  ),
+                ),
               ],
             ),
           ),