fix: storefront login 403, cookie path, double-storefront URLs, and auth redirects

- Extract store/platform context from Referer header for storefront API requests
  (StoreContextMiddleware and PlatformContextMiddleware) so login POST works in
  dev mode where API paths lack /platforms/{code}/ prefix
- Set customer token cookie path to "/" for cross-route compatibility
- Fix double storefront in URLs: replace {{ base_url }}storefront/ with {{ base_url }}
  across all 24 storefront templates
- Fix auth error redirect to include platform prefix and use store_code
- Update seed script to output correct storefront login URLs
- Add 20 new unit tests covering all fixes; fix 9 pre-existing test failures

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/cart/templates/cart/storefront/cart.html

@@ -14,7 +14,7 @@
     <div class="breadcrumb mb-6">
         <a href="{{ base_url }}" class="hover:text-primary">Home</a>
         <span>/</span>
-        <a href="{{ base_url }}storefront/products" class="hover:text-primary">Products</a>
+        <a href="{{ base_url }}products" class="hover:text-primary">Products</a>
         <span>/</span>
         <span class="text-gray-900 dark:text-gray-200 font-medium">Cart</span>
     </div>
@@ -40,7 +40,7 @@
         <p class="text-gray-600 dark:text-gray-400 mb-6">
             Add some products to get started!
         </p>
-        <a href="{{ base_url }}storefront/products" class="inline-block px-6 py-3 bg-primary text-white rounded-lg hover:bg-primary-dark transition-colors">
+        <a href="{{ base_url }}products" class="inline-block px-6 py-3 bg-primary text-white rounded-lg hover:bg-primary-dark transition-colors">
             Browse Products
         </a>
     </div>
@@ -154,7 +154,7 @@
                     Proceed to Checkout
                 </button>
 
-                <a href="{{ base_url }}storefront/products" class="block w-full px-6 py-3 text-center border-2 border-gray-300 dark:border-gray-600 rounded-lg hover:bg-gray-100 dark:hover:bg-gray-700 transition-colors">
+                <a href="{{ base_url }}products" class="block w-full px-6 py-3 text-center border-2 border-gray-300 dark:border-gray-600 rounded-lg hover:bg-gray-100 dark:hover:bg-gray-700 transition-colors">
                     Continue Shopping
                 </a>
 
@@ -309,9 +309,9 @@ document.addEventListener('alpine:init', () => {
 
             if (!token) {
                 // Redirect to login with return URL
-                window.location.href = '{{ base_url }}storefront/account/login?return={{ base_url }}storefront/checkout';
+                window.location.href = '{{ base_url }}account/login?return={{ base_url }}checkout';
             } else {
-                window.location.href = '{{ base_url }}storefront/checkout';
+                window.location.href = '{{ base_url }}checkout';
             }
         }
         };