feat: add language switch button and optimize theme visuals

- Add LanguageSwitch component to all 4 theme layouts (starter/default/dark/minimal)
- Import Noto Sans SC for proper Chinese font rendering
- Fix theme flash on refresh by blocking render until site info loads
- Optimize default theme: violet/blue/cyan palette, SVG icons instead of emoji
- Optimize dark theme: unified emerald accent, reduced particle count, backdrop-blur
- Add Tailwind safelist for dark theme dynamic class names
- Prioritize localStorage in i18n detection so manual language choice persists

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

index.html

@@ -5,7 +5,7 @@
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <link rel="preconnect" href="https://fonts.googleapis.com" />
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
-    <link href="https://fonts.googleapis.com/css2?family=Inter:wght@300;400;500;600;700;800&family=JetBrains+Mono:wght@400;500&display=swap" rel="stylesheet" />
+    <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">