Fix checkout loop by preventing logout during Stripe navigation

Critical fix: Don't clear cart or logout when navigating to Stripe checkout.

The issue was:
1. User clicks 'Continue to payment'
2. API call to /api/checkout succeeds
3. Client redirects to Stripe URL with window.location.href
4. This triggers beforeunload event
5. LogoutOnUnload component logs out and clears cart
6. User is logged out and cart is empty during checkout

Solution:
- Detect when /api/checkout is being called
- Set isCheckingOut flag
- Skip logout/cart-clear in beforeunload if checkout in progress
- User stays logged in and cart persists through Stripe flow

This allows logout on browser close but not during payment navigation.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/components/LogoutOnUnload.tsx

@@ -1,17 +1,40 @@
 'use client';
 
-import { useEffect } from 'react';
+import { useEffect, useRef } from 'react';
 import { useCart } from '@/lib/cartStore';
 
 /**
  * Logs out the user when they close/leave the page
  * Works for both admin and customer sessions
+ * Skips logout when navigating to external checkout (Stripe, etc)
  */
 export default function LogoutOnUnload() {
   const clearCart = useCart((s) => s.clear);
+  const isCheckingOut = useRef(false);
+
+  useEffect(() => {
+    // Mark when checkout is being initiated
+    const origFetch = window.fetch;
+    window.fetch = function(...args) {
+      const url = typeof args[0] === 'string' ? args[0] : (args[0] as Request).url;
+      if (url?.includes('/api/checkout')) {
+        isCheckingOut.current = true;
+      }
+      return origFetch.apply(this, args);
+    };
+
+    return () => {
+      window.fetch = origFetch;
+    };
+  }, []);
 
   useEffect(() => {
     const handleUnload = () => {
+      // Don't logout if we're in the middle of checkout (navigating to Stripe)
+      if (isCheckingOut.current) {
+        return;
+      }
+
       // Clear cart on logout
       clearCart();
       // Use sendBeacon to reliably send logout even when page is closing