feat(desktop): embed BoosAPI Desktop client as a subproject

Vendor the rebranded BoosAPI Desktop client (Tauri + Vue) into metapi as
a monorepo subdirectory under desktop/. Source only — node_modules,
dist, Cargo target and the upstream .git history are excluded.

desktop/ keeps its own independent Cargo workspace (desktop/src-tauri
with crates/*); it is not part of the backend boosapi crate, so a
`cargo check` at the metapi root still compiles only the backend. Build
the desktop app from desktop/src-tauri per desktop/README.md.

Note: app icons are still placeholder assets pending the final BoosAPI
logo. The updater signing keys live outside the repo and are not
included.

desktop/website/docs/.vitepress/theme/index.ts

@@ -0,0 +1,27 @@
+import DefaultTheme from 'vitepress/theme';
+import { useRouter, inBrowser } from 'vitepress';
+import { onMounted } from 'vue';
+import HomeLayout from './HomeLayout.vue';
+import DownloadHero from './DownloadHero.vue';
+import './custom.css';
+
+export default {
+  extends: DefaultTheme,
+  Layout: HomeLayout,
+  enhanceApp({ app }) {
+    app.component('DownloadHero', DownloadHero);
+  },
+  setup() {
+    const router = useRouter();
+
+    onMounted(() => {
+      if (!inBrowser) return;
+      const path = window.location.pathname;
+      if (path !== '/') return;
+      const lang = navigator.language || navigator.languages?.[0] || '';
+      if (/^zh/i.test(lang)) {
+        router.go('/zh/');
+      }
+    });
+  },
+};