fix: 分站加载动画跟随主题色

- 添加 --page-bg/--page-spinner CSS 变量，各主题定义自己的颜色
- body 背景改用 CSS 变量，不再硬编码黑色
- SiteContext 获取站点信息后立即应用主题类到 body
- localStorage 缓存主题类，刷新时在 React 加载前恢复，防止闪烁
- 浅色主题(clean/corporate/claude)加载时显示白色/米色背景+对应色 spinner

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

index.html

@@ -1,5 +1,5 @@
 <!DOCTYPE html>
-<html lang="en" class="dark">
+<html lang="en">
   <head>
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
@@ -8,7 +8,14 @@
     <link href="https://fonts.googleapis.com/css2?family=Inter:wght@300;400;500;600;700;800&family=Noto+Sans+SC:wght@300;400;500;600;700&family=JetBrains+Mono:wght@400;500&display=swap" rel="stylesheet" />
     <title>AI Platform</title>
   </head>
-  <body class="bg-black text-white antialiased">
+  <body class="antialiased">
+    <script>
+      // Restore theme class from cache to prevent flash on reload
+      try {
+        var tc = localStorage.getItem('dist-theme-class');
+        if (tc) document.body.className = tc + ' antialiased';
+      } catch(e) {}
+    </script>
     <div id="root"></div>
     <script type="module" src="/src/main.jsx"></script>
   </body>