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/Core/CoreGateway.swift

@@ -47,3 +47,4 @@ protocol CoreGateway: AnyObject {
 	func receiverRequests(transferId: UInt64) async -> Result<[ReceiverRequestModel], Error>
 	func respondReceiverRequest(requestId: String, accepted: Bool, reason: String?) async -> Result<Void, Error>
 	func refresh() async -> Result<Void, Error>
+}