style: 重新设计语言切换按钮 — SVG 地球图标 + CN/EN 双标签

- emoji 🌐 替换为精致 SVG 线条地球图标
- 按钮改为圆角胶囊样式，透明底 + 细边框
- 显示 CN/EN 双标签（当前语言高亮，另一个淡显）
- hover 时整体变蓝色主题色

common/common.css

@@ -133,32 +133,62 @@
 .lang-switch {
   display: flex;
   align-items: center;
-  gap: 4px;
-  padding: 6px 12px;
-  margin-left: 12px;
-  border-radius: 6px;
+  gap: 6px;
+  padding: 5px 14px;
+  margin-left: 16px;
+  border-radius: 20px;
   cursor: pointer;
-  color: rgba(220, 228, 240, 0.85);
-  font-size: 13px;
+  color: rgba(203, 213, 225, 0.7);
+  font-size: 12px;
   font-weight: 500;
-  transition: background 0.15s ease, color 0.15s ease;
+  letter-spacing: 0.5px;
+  transition: all 0.25s ease;
   user-select: none;
-  background: rgba(255, 255, 255, 0.06);
-  border: 1px solid rgba(255, 255, 255, 0.08);
+  background: transparent;
+  border: 1px solid rgba(148, 163, 184, 0.15);
 }
 
 .lang-switch:hover {
-  background: rgba(255, 255, 255, 0.12);
-  color: #fff;
+  background: rgba(56, 189, 248, 0.08);
+  border-color: rgba(56, 189, 248, 0.3);
+  color: #38BDF8;
 }
 
 .lang-switch .lang-icon {
-  font-size: 15px;
-  line-height: 1;
+  width: 16px;
+  height: 16px;
+  flex-shrink: 0;
+  opacity: 0.7;
+  transition: opacity 0.25s ease;
+}
+
+.lang-switch:hover .lang-icon {
+  opacity: 1;
 }
 
 .lang-switch .lang-text {
   font-weight: 600;
+  color: rgba(226, 232, 240, 0.95);
+  transition: color 0.25s ease;
+}
+
+.lang-switch:hover .lang-text {
+  color: #38BDF8;
+}
+
+.lang-switch .lang-sep {
+  color: rgba(148, 163, 184, 0.3);
+  font-weight: 300;
+}
+
+.lang-switch .lang-alt {
+  color: rgba(148, 163, 184, 0.45);
+  font-weight: 400;
+  transition: color 0.25s ease;
+}
+
+.lang-switch:hover .lang-alt {
+  color: rgba(56, 189, 248, 0.6);
 }
 
 .breadcrumb {