feat(auth): redesign auth screens with clean form style and add welcome page

- Replace split/responsive layout with unified clean form design
- Update LoginScreen, RegisterScreen, and ForgotPasswordScreen styles
- Add WelcomeScreen as new entry point for unauthenticated users
- Redirect unauthenticated users to /welcome instead of /login
- Add hrefAuthWelcome() navigation helper

BREAKING CHANGE: Auth screens no longer support split layout, unified mobile-first design

src/screens/auth/index.ts

@@ -6,3 +6,4 @@
 export { LoginScreen } from './LoginScreen';
 export { RegisterScreen } from './RegisterScreen';
 export { ForgotPasswordScreen } from './ForgotPasswordScreen';
+export { WelcomeScreen } from './WelcomeScreen';