Connect login/register forms to BetterAuth signIn.email() and signUp.email()
with error display, loading states, and disabled inputs during submission.
Add AuthGuard (redirects unauthenticated users to /login) on dashboard routes
and GuestGuard (redirects authenticated users to /chat) on auth routes.
Sign out in topbar now redirects to /login.
Refs #27
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
