fix: update Antigravity user agent version to 1.15.8

Fixes #308 - 'This version of Antigravity is no longer supported' error

The API now enforces a version check on the User-Agent string. Updated:
- ANTIGRAVITY_VERSIONS array from 1.10.0-1.12.1 to 1.14.0-1.15.8
- collectCurrentFingerprint() hardcoded version from 1.11.5 to 1.15.8