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/src/main.tsx

@@ -1,16 +1,16 @@
 import { StrictMode } from "react";
 import { createRoot } from "react-dom/client";
-import { HashRouter } from "react-router-dom";
+import { BrowserRouter } from "react-router-dom";
 import { LangProvider } from "./i18n";
 import App from "./App";
 import "./index.css";
 
 createRoot(document.getElementById("root")!).render(
   <StrictMode>
-    <HashRouter>
+    <BrowserRouter basename="/KimiSwitch">
       <LangProvider>
         <App />
       </LangProvider>
-    </HashRouter>
+    </BrowserRouter>
   </StrictMode>,
 );