iOS was missing NSFaceIDUsageDescription — Face ID prompts will
crash without it (required by Apple since iOS 11). Added with
user-facing reason string scoped to health data privacy.
Android was missing USE_BIOMETRIC and USE_FINGERPRINT permissions —
biometric_local_auth plugin requires both for API 28+/legacy compat.
The Flutter implementation was already complete:
- BiometricService (services/biometric_service.dart)
- AuthGate lock screen (core/auth_gate.dart)
- Settings toggle with policy control (features/settings/)
- Wired into main.dart
These permission declarations were the only missing pieces
blocking Face ID / fingerprint from working on device.