Add PWA install support for mobile and desktop

Register service worker, install prompt UI, iOS meta tags, and document install steps in DEPLOY.md.

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

.vitepress/theme/index.ts

@@ -0,0 +1,12 @@
+import DefaultTheme from 'vitepress/theme'
+import { h } from 'vue'
+import InstallApp from './InstallApp.vue'
+
+export default {
+  extends: DefaultTheme,
+  Layout: () => {
+    return h(DefaultTheme.Layout, null, {
+      'layout-bottom': () => h(InstallApp),
+    })
+  },
+}