style: 语言切换改纯文字 + 移到登录注册左边

- 去掉边框、背景、圆角，纯文字 CN/EN
- 去掉 SVG 地球图标
- 从右侧移到登录注册按钮左边

common/common.css

@@ -133,24 +133,21 @@
 .lang-switch {
   display: flex;
   align-items: center;
-  gap: 6px;
-  padding: 5px 14px;
-  margin-left: 16px;
-  border-radius: 20px;
+  gap: 3px;
+  padding: 4px 0;
+  margin-right: 16px;
   cursor: pointer;
   color: rgba(203, 213, 225, 0.7);
-  font-size: 12px;
+  font-size: 13px;
   font-weight: 500;
-  letter-spacing: 0.5px;
-  transition: all 0.25s ease;
+  letter-spacing: 0.3px;
+  transition: color 0.25s ease;
   user-select: none;
   background: transparent;
-  border: 1px solid rgba(148, 163, 184, 0.15);
+  border: none;
 }
 
 .lang-switch:hover {
-  background: rgba(56, 189, 248, 0.08);
-  border-color: rgba(56, 189, 248, 0.3);
   color: #38BDF8;
 }