Features:
- FCM Web Push (Chrome, Firefox, Edge) - same API as mobile
- Safari Web Push (macOS) via APNS with web. topic prefix

New functions:
- PushX.FCM.web_notification/4 - Create web push payloads
- PushX.FCM.send_web/5 - Convenience function for web notifications
- PushX.APNS.web_notification/4 - Safari web push with URL args
- PushX.APNS.web_notification_with_data/5 - Safari web push with data

Changes:
- FCM token validation accepts shorter web tokens (min 20 chars)
- Updated Finch to ~> 0.21
- Updated documentation with Web Push examples

Tests: 205 tests, 0 failures