refactor: rename agent type selection label to professional role for clarity

src/app/(auth)/signup/page.tsx

@@ -288,7 +288,7 @@ export default function SignUpPage() {
                 backgroundSize: '1.25rem',
               }}
             >
-              <option value="">Select Agent Type</option>
+              <option value="">Select Professional Role</option>
               {agentTypes.map((type) => (
                 <option key={type.id} value={type.id}>
                   {type.name}