fix(apple): realign UI layer with regenerated core bindings

- map the new VnidropError cases (DeviceUnavailable, OfferTimeout,
  RelayPolicyIncompatible, ProtocolIncompatible) to existing catalog keys
- drop the stale .map(\.share) now that sharePickedFiles returns Share
- remove the duplicate .transfersChanged pattern in the signal switch
- replace the deprecated String(cString:) sysctl decode
- close the CoreGateway protocol declaration

apple/VniDrop/Features/Notifications/TransferNotificationCoordinator.swift

@@ -111,7 +111,7 @@ final class TransferNotificationCoordinator: ObservableObject {
 				switch signal {
 				case .receiverHistoryChanged(let transferId), .transfersChanged(let transferId):
 					Task { await self.syncReceivers(transferId: transferId) }
-				case .approvalChanged, .transfersChanged:
+				case .approvalChanged:
 					break
 				}
 			}