rebrand cli oauth success page to match kernel.sh

- switch from dark theme to light beige (#f2f0e7) background
- replace Commit Mono with Inter Light (300)
- remove 100KB base64 background image overlay
- add inline krnl. wordmark SVG
- hard corners on checkmark container (0.5px solid border)
- kernel green (#81b300) checkmark instead of purple
- lowercase text-transform throughout

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: masnwilliams