修复 register-advert 与 img-responsive 高度不一致问题

- 为 .register-advert 添加 overflow: hidden 防止溢出
- 设置链接为 block 并占满容器高度
- 图片使用固定高度 75px 与容器一致
- 使用 object-fit: cover 保持图片比例

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

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

common/common.css

@@ -306,17 +306,24 @@
   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; */
 }
 
 .register-advert a {
+  display: block;
+  width: 100%;
+  height: 100%;
   text-align: center;
 }
 
 .register-advert a img {
-  max-width: 100%;
+  width: 100%;
+  height: 75px;
+  object-fit: cover;
+  display: block;
   margin: 0 auto;
 }