fix(signup): remove duplicate role_key field from professional registration

The register() request body sent both profession and role_key with the
same value. The backend's registration DTO aliases role_key onto the same
field as profession, so serde_json rejected the payload outright with a
"duplicate field" deserialization error — professional signup was
returning 422 for every role in production.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/routes/signup/professional.tsx

@@ -173,7 +173,6 @@ export default function ProfessionalSignupRoute() {
           phone: "",
           intent: "professional",
           profession: profession().toUpperCase(),
-          role_key: profession().toUpperCase(),
         }),
       });