Improve login form validation and enhance e2e test reliability

- Update `LoginForm` to use `onBlur` mode and `onChange` revalidation for better UX.
- Enhance e2e tests to verify field interactivity, ensure visibility of submit button, and improve error validation reliability across browsers.

frontend/src/components/auth/LoginForm.tsx

@@ -82,6 +82,8 @@ export function LoginForm({
 
   const form = useForm<LoginFormData>({
     resolver: zodResolver(loginSchema),
+    mode: 'onBlur',
+    reValidateMode: 'onChange',
     defaultValues: {
       email: '',
       password: '',