feat: post-signup onboarding wizard (dietary/allergen prefs, notifications, skippable) (v0.77.0)

New accounts land on a 3-step wizard after signup — welcome, dietary preferences & allergens, notification opt-in — skippable at every step, shown once via a users.onboardingCompletedAt gate in the (app) layout. Existing accounts are backfilled as already-onboarded.

Also gives the allergen-preferences step a real write path: user_allergens previously only had a GDPR-export read.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/app/(app)/layout.tsx

@@ -1,6 +1,19 @@
+import { headers } from "next/headers";
+import { redirect } from "next/navigation";
 import { Nav } from "@/components/layout/nav";
+import { auth } from "@/lib/auth/server";
+import { db, users, eq } from "@epicure/db";
+
+export default async function AppLayout({ children }: { children: React.ReactNode }) {
+  const session = await auth.api.getSession({ headers: await headers() });
+  if (session) {
+    const dbUser = await db.query.users.findFirst({
+      where: eq(users.id, session.user.id),
+      columns: { onboardingCompletedAt: true },
+    });
+    if (!dbUser?.onboardingCompletedAt) redirect("/onboarding");
+  }
 
-export default function AppLayout({ children }: { children: React.ReactNode }) {
   return (
     <>
       <Nav />