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/checkout/templates/checkout/storefront/checkout.html

@@ -11,10 +11,10 @@
     {# Breadcrumbs #}
     <nav class="mb-6" aria-label="Breadcrumb">
         <ol class="flex items-center space-x-2 text-sm text-gray-500 dark:text-gray-400">
-            <li><a href="{{ base_url }}storefront/" class="hover:text-primary">Home</a></li>
+            <li><a href="{{ base_url }}" class="hover:text-primary">Home</a></li>
             <li class="flex items-center">
                 <span class="h-4 w-4 mx-2" x-html="$icon('chevron-right', 'h-4 w-4')"></span>
-                <a href="{{ base_url }}storefront/cart" class="hover:text-primary">Cart</a>
+                <a href="{{ base_url }}cart" class="hover:text-primary">Cart</a>
             </li>
             <li class="flex items-center">
                 <span class="h-4 w-4 mx-2" x-html="$icon('chevron-right', 'h-4 w-4')"></span>
@@ -36,7 +36,7 @@
         <span class="mx-auto h-12 w-12 text-gray-400 mb-4 block" x-html="$icon('shopping-bag', 'h-12 w-12 mx-auto')"></span>
         <h3 class="text-xl font-semibold text-gray-700 dark:text-gray-200 mb-2">Your cart is empty</h3>
         <p class="text-gray-500 dark:text-gray-400 mb-6">Add some products before checking out.</p>
-        <a href="{{ base_url }}storefront/products" class="inline-block px-6 py-3 text-white rounded-lg transition-colors" style="background-color: var(--color-primary)">
+        <a href="{{ base_url }}products" class="inline-block px-6 py-3 text-white rounded-lg transition-colors" style="background-color: var(--color-primary)">
             Browse Products
         </a>
     </div>
@@ -900,7 +900,7 @@ function checkoutPage() {
                 console.log('[CHECKOUT] Order placed:', order.order_number);
 
                 // Redirect to confirmation page
-                window.location.href = '{{ base_url }}storefront/order-confirmation?order=' + order.order_number;
+                window.location.href = '{{ base_url }}order-confirmation?order=' + order.order_number;
 
             } catch (error) {
                 console.error('[CHECKOUT] Error placing order:', error);