Fix PWA install by registering service worker in theme

VitePress did not auto-inject SW registration; manual registerSW enables true app install on HTTPS.

Co-authored-by: Cursor <cursoragent@cursor.com>

.vitepress/theme/index.ts

@@ -2,6 +2,12 @@ import DefaultTheme from 'vitepress/theme'
 import { h } from 'vue'
 import InstallApp from './InstallApp.vue'
 
+if (typeof window !== 'undefined') {
+  import('virtual:pwa-register').then(({ registerSW }) => {
+    registerSW({ immediate: true })
+  })
+}
+
 export default {
   extends: DefaultTheme,
   Layout: () => {