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/components/pwa/sw-register.tsx

@@ -0,0 +1,13 @@
+"use client";
+
+import { useEffect } from "react";
+
+export function SwRegister() {
+  useEffect(() => {
+    if ("serviceWorker" in navigator) {
+      navigator.serviceWorker.register("/sw.js").catch(console.error);
+    }
+  }, []);
+
+  return null;
+}