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/billing/templates/billing/admin/subscription-tiers.html

@@ -352,7 +352,7 @@
             x-transition:leave="transform transition ease-in-out duration-300"
             x-transition:leave-start="translate-x-0"
             x-transition:leave-end="translate-x-full"
-            class="w-screen max-w-lg"
+            class="w-screen max-w-lg h-full"
         >
             <div class="flex h-full flex-col bg-white dark:bg-gray-800 shadow-xl">
                 <!-- Header -->