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>

src/App.jsx

@@ -13,8 +13,9 @@ const Topup = lazy(() => import('./pages/Topup'));
 const Logs = lazy(() => import('./pages/Logs'));
 
 const Loading = () => (
-  <div className="flex items-center justify-center min-h-screen">
-    <div className="w-8 h-8 border-2 border-brand-500/30 border-t-brand-500 rounded-full animate-spin" />
+  <div className="flex items-center justify-center min-h-screen" style={{ background: 'var(--page-bg)' }}>
+    <div className="w-8 h-8 rounded-full animate-spin"
+      style={{ border: '2px solid var(--page-spinner-track)', borderTopColor: 'var(--page-spinner)' }} />
   </div>
 );