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/vite.config.ts

@@ -1,11 +1,30 @@
-import { defineConfig } from "vite";
+import { defineConfig, type Plugin } from "vite";
 import react from "@vitejs/plugin-react";
+import { copyFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+
+/**
+ * GitHub Pages SPA fallback: any deep path (e.g. /KimiSwitch/features) is
+ * served by 404.html; duplicating the built index.html there lets the
+ * BrowserRouter boot and render the right route. This keeps every page
+ * indexable as a real URL instead of a `#/` fragment.
+ */
+function generate404(): Plugin {
+  return {
+    name: "generate-404",
+    apply: "build",
+    closeBundle() {
+      const dist = fileURLToPath(new URL("./dist", import.meta.url));
+      copyFileSync(`${dist}/index.html`, `${dist}/404.html`);
+    },
+  };
+}
 
 // GitHub Pages subpath: billowliu2.github.io/KimiSwitch/
 export default defineConfig({
   base: "/KimiSwitch/",
-  plugins: [react()],
+  plugins: [react(), generate404()],
   resolve: {
     alias: { "@": "/src" },
   },
-});
+});