Implement ordering MVP domain layer per spec (issue #1)

Scaffolds the TanStack Start + Drizzle/Postgres + Better Auth app and
implements the core ordering domain: menu browsing, guest/account order
placement with online payment, per-Order-Item independent cooking and
cancellation, dynamic Machine-Tag pool assignment, and multi-step recipes
(see docs/adr/0001-0003). Adds a guest-access-token / session-based
ownership check on every read/mutation so an order can't be viewed,
cancelled, or picked up by anyone but its owner.

Covered by 14 tests running the domain layer directly against a real,
ephemeral Postgres (testcontainers) — createServerFn couldn't be called
outside a real request context, so tests exercise the domain functions the
server functions thinly wrap instead of the server functions themselves.

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

src/integrations/better-auth/header-user.tsx

@@ -0,0 +1,39 @@
+import { authClient } from '#/lib/auth-client'
+
+const BetterAuthHeader = () => {
+  const { data: session, isPending } = authClient.useSession()
+
+  if (isPending) {
+    return (
+      <div className="h-8 w-8 bg-neutral-100 dark:bg-neutral-800 animate-pulse" />
+    )
+  }
+
+  if (session?.user) {
+    return (
+      <div className="flex items-center gap-2">
+        {session.user.image ? (
+          <img src={session.user.image} alt="" className="h-8 w-8" />
+        ) : (
+          <div className="h-8 w-8 bg-neutral-100 dark:bg-neutral-800 flex items-center justify-center">
+            <span className="text-xs font-medium text-neutral-600 dark:text-neutral-400">
+              {session.user.name.charAt(0).toUpperCase() || 'U'}
+            </span>
+          </div>
+        )}
+        <button
+          onClick={() => {
+            void authClient.signOut()
+          }}
+          className="flex-1 h-9 px-4 text-sm font-medium bg-white dark:bg-neutral-900 text-neutral-900 dark:text-neutral-50 border border-neutral-300 dark:border-neutral-700 hover:bg-neutral-50 dark:hover:bg-neutral-800 transition-colors"
+        >
+          Sign out
+        </button>
+      </div>
+    )
+  }
+
+  return null
+}
+
+export default BetterAuthHeader