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/App.tsx

@@ -8,6 +8,10 @@ import Download from "./sections/Download";
 import Privacy from "./sections/Privacy";
 import CTA from "./sections/CTA";
 import Footer from "./components/Footer";
+import { useLang, type Dict } from "./i18n";
+
+const SITE_URL = "https://billowliu2.github.io/KimiSwitch";
+type SeoKey = keyof Dict["seo"];
 
 function ScrollToTop() {
   const { pathname } = useLocation();
@@ -17,6 +21,47 @@ function ScrollToTop() {
   return null;
 }
 
+/** Update per-page title / description / OG / canonical for SEO. */
+function RouteMeta() {
+  const { pathname } = useLocation();
+  const { t } = useLang();
+
+  useEffect(() => {
+    const page = pathname.replace(/\/+$/, "");
+    const seoKey: SeoKey =
+      page === "/features" || page === "/changelog" || page === "/download"
+        ? (page.slice(1) as SeoKey)
+        : "home";
+    const m = t.seo[seoKey];
+    const url = `${SITE_URL}${page === "" ? "/" : page}`;
+
+    document.title = m.title;
+    setMeta("name", "description", m.description);
+    setMeta("property", "og:title", m.title);
+    setMeta("property", "og:description", m.description);
+    setMeta("property", "og:url", url);
+    setMeta("name", "twitter:title", m.title);
+    setMeta("name", "twitter:description", m.description);
+
+    const canon = document.querySelector('link[rel="canonical"]');
+    if (canon) canon.setAttribute("href", url);
+  }, [pathname, t]);
+
+  return null;
+}
+
+function setMeta(attr: "name" | "property", key: string, content: string) {
+  let el = document.head.querySelector<HTMLMetaElement>(
+    `meta[${attr}="${key}"]`,
+  );
+  if (!el) {
+    el = document.createElement("meta");
+    el.setAttribute(attr, key);
+    document.head.appendChild(el);
+  }
+  el.setAttribute("content", content);
+}
+
 const homePage = (
   <>
     <Hero />
@@ -30,6 +75,7 @@ export default function App() {
       <Nav />
       <main className="flex-1">
         <ScrollToTop />
+        <RouteMeta />
         <Routes>
           <Route path="/" element={homePage} />
           <Route