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/lib/push.ts

@@ -0,0 +1,33 @@
+import webpush from "web-push";
+import { db, pushSubscriptions, eq } from "@epicure/db";
+
+webpush.setVapidDetails(
+  "mailto:contact@epicure.app",
+  process.env.NEXT_PUBLIC_VAPID_PUBLIC_KEY!,
+  process.env.VAPID_PRIVATE_KEY!
+);
+
+export async function sendPushNotification(
+  userId: string,
+  notification: { title: string; body: string; url?: string }
+): Promise<void> {
+  const subs = await db
+    .select()
+    .from(pushSubscriptions)
+    .where(eq(pushSubscriptions.userId, userId));
+
+  const payload = JSON.stringify({
+    title: notification.title,
+    body: notification.body,
+    url: notification.url ?? "/",
+  });
+
+  await Promise.allSettled(
+    subs.map((sub) =>
+      webpush.sendNotification(
+        { endpoint: sub.endpoint, keys: { p256dh: sub.p256dh, auth: sub.auth } },
+        payload
+      )
+    )
+  );
+}