Implement LocalAuthIntegration platform adapter

Define an abstract class ILocalAuthIntegration in the domain layer, and place the concrete LocalAuthIntegrationImpl in the infrastructure/platform layer. This way BiometricAuthService depends only on the interface and the Riverpod provider can swap in a fake during testing. Use the Result type already established in task-002 — do not introduce a second Result type. For the BiometricType mapping, use a switch expression (Dart 3) for exhaustive mapping, with a default returning null, then filter nulls out of the list.

Be careful on iOS: canCheckBiometrics can return true even when Face ID is not enrolled but hardware is present — isDeviceSupported() is the more reliable gate. On Android, canCheckBiometrics covers both fingerprint and face sensors.

Unit tests using flutter_test with a mocked LocalAuthentication (implement using mockito or manual fake). Cover: (1) isAvailable returns false when isDeviceSupported returns false, (2) isAvailable returns false when canCheckBiometrics returns false, (3) isAvailable returns true when both return true, (4) getAvailableBiometrics maps BiometricType.face to domain face, (5) getAvailableBiometrics maps BiometricType.fingerprint to domain fingerprint, (6) getAvailableBiometrics drops unknown types, (7) authenticate returns Result.success on true return from local_auth, (8) authenticate returns Result.failure(LocalAuthFailure.cancelled) when PlatformException with cancelled code is thrown, (9) each exception type maps to the expected domain failure. All tests must be pure Dart (no device required).