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

app/welcome.tsx

@@ -0,0 +1,5 @@
+import { WelcomeScreen } from '../src/screens/auth';
+
+export default function WelcomeRoute() {
+  return <WelcomeScreen />;
+}