Fix login flow and OAuth preview URLs
- Add login modal to Header with handle input
- Replace non-functional login link with modal trigger button
- Configure preview environment with correct PUBLIC_BASE_URL
- Fix OAuth redirects to use preview URLs instead of production
This allows users to log in from any page without navigation,
and enables testing OAuth on feature branches.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>