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

public/header.html

@@ -54,8 +54,10 @@
       </div>
       <!-- 语言切换（始终显示） -->
       <div class="lang-switch nav-desktop-link" id="langSwitch" onclick="i18n.toggle()">
-        <span class="lang-icon">🌐</span>
-        <span class="lang-text">中</span>
+        <svg class="lang-icon" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.8" stroke-linecap="round" stroke-linejoin="round"><circle cx="12" cy="12" r="10"/><path d="M2 12h20"/><path d="M12 2a15.3 15.3 0 0 1 4 10 15.3 15.3 0 0 1-4 10 15.3 15.3 0 0 1-4-10 15.3 15.3 0 0 1 4-10z"/></svg>
+        <span class="lang-text">CN</span>
+        <span class="lang-sep">/</span>
+        <span class="lang-alt">EN</span>
       </div>
       <!-- 移动端汉堡菜单按钮 -->
       <button class="mobile-menu-toggle" id="mobileMenuToggle" aria-label="Toggle navigation">