site: drop PWA manifest from marketing site (SEO site, not an app)

Remove the web-app manifest link + manifest.ts route so bargevasat.ir no longer
triggers an "install/add to home screen" prompt. It's a plain marketing/SEO
site now. Only the game app (app.bargevasat.ir) remains a PWA.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

site/app/layout.tsx

@@ -41,7 +41,8 @@ export const metadata: Metadata = {
     images: ["/og.png"],
   },
   icons: { icon: "/icon.svg", apple: "/icon.svg" },
-  manifest: "/manifest.webmanifest",
+  // No web-app manifest on the marketing site — it's a plain SEO site, not an
+  // installable PWA. Only the game app (app.bargevasat.ir) is a PWA.
 };
 
 export const viewport: Viewport = {