The connect.garmin.com/modern/proxy URL returns HTML (website) instead
of JSON API responses. Garth library uses connectapi.garmin.com subdomain
directly, which is the actual API endpoint.
- Change base URL from connect.garmin.com/modern/proxy to connectapi.garmin.com
- Update User-Agent to match garth library: GCM-iOS-5.19.1.2
- Factor out headers into getGarminHeaders() to avoid duplication
- Remove NK header (not needed when using connectapi subdomain)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>