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>

src/components/LanguageSwitch.jsx

@@ -0,0 +1,25 @@
+import { useTranslation } from 'react-i18next';
+
+export default function LanguageSwitch({ className = '' }) {
+  const { i18n } = useTranslation();
+  const isZh = i18n.language?.startsWith('zh');
+
+  const toggle = () => {
+    i18n.changeLanguage(isZh ? 'en' : 'zh');
+  };
+
+  return (
+    <button
+      onClick={toggle}
+      className={`flex items-center gap-1.5 px-2.5 py-1 rounded-md text-xs transition-colors ${className}`}
+      aria-label="Switch language"
+    >
+      <svg className="w-3.5 h-3.5" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth={1.5}>
+        <path strokeLinecap="round" strokeLinejoin="round" d="M12 21a9 9 0 1 0 0-18 9 9 0 0 0 0 18Z" />
+        <path strokeLinecap="round" strokeLinejoin="round" d="M3.6 9h16.8M3.6 15h16.8" />
+        <path strokeLinecap="round" strokeLinejoin="round" d="M12 3a15.3 15.3 0 0 1 4 9 15.3 15.3 0 0 1-4 9 15.3 15.3 0 0 1-4-9 15.3 15.3 0 0 1 4-9Z" />
+      </svg>
+      <span>{isZh ? 'EN' : '中文'}</span>
+    </button>
+  );
+}