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/index.tsx

@@ -7,5 +7,5 @@ export default function Index() {
   if (isAuthenticated) {
     return <Redirect href="/home" />;
   }
-  return <Redirect href="/login" />;
+  return <Redirect href="/welcome" />;
 }