feat(website): SEO 升级 — BrowserRouter + 每页独立 meta

- HashRouter → BrowserRouter（basename /KimiSwitch），GitHub Pages 用 404.html SPA 回退，/features /download /changelog 变为可收录的独立 URL
- 每页独立 title / description / OG / canonical（RouteMeta 随语言切换）
- og:image 改为绝对地址；sitemap 更新为干净路径

website/package.json

@@ -17,6 +17,7 @@
     "react-router-dom": "^7.18.2"
   },
   "devDependencies": {
+    "@types/node": "^26.1.2",
     "@types/react": "^18.3.3",
     "@types/react-dom": "^18.3.0",
     "@vitejs/plugin-react": "^4.3.1",