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/pwa.mts

@@ -3,7 +3,7 @@ import { VitePWA } from 'vite-plugin-pwa'
 export function createPwaPlugin() {
   return VitePWA({
     registerType: 'autoUpdate',
-    injectRegister: 'auto',
+    injectRegister: null,
     manifestFilename: 'site.webmanifest',
     includeAssets: [
       'favicon.ico',
@@ -14,6 +14,7 @@ export function createPwaPlugin() {
       'icon-512.png',
     ],
     manifest: {
+      id: '/',
       name: '道德经 · 传统文化典籍',
       short_name: '道德经',
       description: '传统文化典籍资料库',