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>

server/public/login.html

@@ -5,6 +5,9 @@
   <meta name="viewport" content="width=device-width, initial-scale=1.0" />
   <meta name="theme-color" content="#0f3460" />
   <meta name="apple-mobile-web-app-title" content="道德经" />
+  <meta name="apple-mobile-web-app-capable" content="yes" />
+  <meta name="mobile-web-app-capable" content="yes" />
+  <meta name="apple-mobile-web-app-status-bar-style" content="black-translucent" />
   <link rel="icon" href="/favicon.ico" sizes="any" />
   <link rel="shortcut icon" href="/favicon.ico" />
   <link rel="icon" href="/favicon.svg" type="image/svg+xml" />