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/index.js

@@ -104,6 +104,8 @@ const PUBLIC_PATHS = new Set([
 function isPublicPath(reqPath) {
   if (PUBLIC_PATHS.has(reqPath)) return true
   if (reqPath.startsWith('/assets/')) return true
+  if (reqPath === '/sw.js' || reqPath.endsWith('.webmanifest')) return true
+  if (/^\/workbox-[\w-]+\.js$/.test(reqPath)) return true
   return false
 }