Add validation feedback in Create Test Suite modal

Create Suite silently fails when name or base_url is empty. Add visible validation.\n\nFiles:\n- frontend/src/pages/TestSuites.tsx\n\nTasks:\n- Show inline validation messages for missing name/base_url\n- Disable Save button until valid\n\nAcceptance Criteria:\n- Clear error messages for missing required fields\n- Save cannot be triggered with invalid input