feat(pwa): service worker, offline page, web push notifications

PWA manifest, sw.js with offline cache. VAPID web-push via lib/push.ts.
Push subscribe button (client). Push fired on recipe comment creation.
VAPID keys configurable via admin site settings.

apps/web/app/offline/page.tsx

@@ -0,0 +1,18 @@
+import Link from "next/link";
+
+export default function OfflinePage() {
+  return (
+    <div className="flex min-h-screen flex-col items-center justify-center gap-4 p-8 text-center">
+      <h1 className="text-2xl font-semibold">You're offline</h1>
+      <p className="max-w-sm text-muted-foreground">
+        Your recently visited recipes are available. Connect to internet to load new content.
+      </p>
+      <Link
+        href="/recipes"
+        className="mt-2 underline underline-offset-4 hover:text-primary"
+      >
+        Back to my recipes
+      </Link>
+    </div>
+  );
+}