- Export setEnvironmentMode + BootpayEnvironmentMode from package root
  (root index.tsx had not re-exported them since they landed in src/index.tsx)
- Bump package to 13.15.0; sync SDK_VERSION runtime constants in
  Bootpay.tsx and BootpayWidget.tsx so webview reports the real package version
- CHANGELOG covers the env API + example client_key/.env cleanup +
  iOS Info.plist URL scheme realignment to bootpayReactNativeExample

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>