Remove N3C API dependencies and add bundle caching

- Remove /whoami and /check-token endpoints (N3C API no longer accessible)
- Remove UsageReport component and /usage route
- Remove Database Refresh section from About page
- Cache bundle data to JSON file for offline use
- Update bundle endpoints to use cached data instead of N3C API
- Add /bundle-download endpoint to download all codesets in a bundle as zip
- Update About page with bundle selector and download buttons

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Sigfried