Clear guest order refs and basket on sign out

Guest order refs and the basket both live in localStorage independent
of the auth session, so they'd otherwise linger for the next person to
use the browser. Add clearGuestOrderRefs (mirroring clearGuestIdentity)
and call it alongside the existing basket clear() from the sign-out
handler.

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

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

@@ -2,6 +2,8 @@ import { Link, useRouterState } from '@tanstack/react-router'
 import { useEffect, useRef, useState } from 'react'
 
 import { authClient } from '#/lib/auth-client'
+import { useBasket } from '#/lib/basket'
+import { clearGuestOrderRefs } from '#/lib/guestOrders'
 
 import styles from './header-user.module.scss'
 
@@ -63,6 +65,7 @@ const LogOutIcon = () => (
 
 const BetterAuthHeader = () => {
   const { data: session, isPending } = authClient.useSession()
+  const { clear: clearBasket } = useBasket()
   const [open, setOpen] = useState(false)
   const wrapperRef = useRef<HTMLDivElement>(null)
   const pathname = useRouterState({
@@ -145,6 +148,8 @@ const BetterAuthHeader = () => {
                 className={styles.menuButton}
                 onClick={() => {
                   setOpen(false)
+                  clearGuestOrderRefs()
+                  clearBasket()
                   void authClient.signOut()
                 }}
               >