修复 register-advert padding 导致的高度错位问题

- 移除 #index .register-advert 的 padding: 40px 0
- 设置 padding: 0 和 height: auto 让容器自适应
- 图片使用 height: auto 和 object-fit: contain
- 移除 common.css 中的固定高度限制

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

common/common.css

@@ -304,9 +304,7 @@
 
 .register-advert {
   width: 100%;
-  height: 75px;
   background: linear-gradient(90deg, #EF4C1D 0%, #F41A10 100%);
-  overflow: hidden;
   /* background-image: url(../assets/img/index/register@2x.png);
   background-repeat: no-repeat;
   background-size: cover; */
@@ -315,14 +313,11 @@
 .register-advert a {
   display: block;
   width: 100%;
-  height: 100%;
   text-align: center;
 }
 
 .register-advert a img {
   width: 100%;
-  height: 75px;
-  object-fit: cover;
   display: block;
   margin: 0 auto;
 }