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/navigation/hrefs.ts

@@ -156,6 +156,10 @@ export function hrefAuthForgot(): string {
   return '/forgot-password';
 }
 
+export function hrefAuthWelcome(): string {
+  return '/welcome';
+}
+
 // ==================== Materials (学习资料) ====================
 
 export function hrefMaterials(): string {