New slot 'narrative' enables split-screen layout (50/50 desktop, collapses
to single column on mobile <900px).
Backward compatibility:
- Pages that don't pass a narrative slot still render the original
centered-card layout with siteName + logo + copyright
- ForgotPassword, ResetPassword, EmailVerify unaffected
To be used in Tasks 8 and 9 (LoginView, RegisterView).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
