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/exceptions/handler.py

@@ -434,14 +434,19 @@ def _redirect_to_login(request: Request) -> RedirectResponse:
 
         base_url = "/"
         if access_method == "path" and store:
-            full_prefix = (
-                store_context.get("full_prefix", "/store/")
-                if store_context
-                else "/store/"
-            )
-            base_url = f"{full_prefix}{store.subdomain}/"
+            platform = getattr(request.state, "platform", None)
+            platform_original_path = getattr(request.state, "platform_original_path", None)
+            if platform and platform_original_path and platform_original_path.startswith("/platforms/"):
+                base_url = f"/platforms/{platform.code}/storefront/{store.store_code}/"
+            else:
+                full_prefix = (
+                    store_context.get("full_prefix", "/storefront/")
+                    if store_context
+                    else "/storefront/"
+                )
+                base_url = f"{full_prefix}{store.store_code}/"
 
-        login_url = f"{base_url}storefront/account/login"
+        login_url = f"{base_url}account/login"
         logger.debug(f"Redirecting to {login_url}")
         return RedirectResponse(url=login_url, status_code=302)
     # Fallback to root for unknown contexts (PLATFORM)