Add artdag_url() helper for cross-app navigation

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

infrastructure/urls.py

@@ -45,6 +45,10 @@ def account_url(path: str = "/") -> str:
     return app_url("account", path)
 
 
+def artdag_url(path: str = "/") -> str:
+    return app_url("artdag", path)
+
+
 def page_cart_url(page_slug: str, path: str = "/") -> str:
     if not path.startswith("/"):
         path = "/" + path