feat: improve login dialog styling and add legal links
- Update dialog to use elevated surface background with larger border radius
- Add close button with grain-icon and escape key support
- Add links to Terms and Privacy Policy
- Adjust input background to match dialog surface
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>