feat(apple): expose device history through the core gateway

Adds contact, pairing, and offer models plus the gateway surface the feature
models will use. Contacts and offers are endpoint-scoped events with no
transfer id, so they get their own coalesced signals.

DeviceContact.displayName prefers the local label over the name the peer
claims, and carries a short endpoint fingerprint for telling apart devices
using the same name.

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:
+				case .approvalChanged, .contactsChanged, .offersChanged:
 					break
 				}
 			}

apple/VniDrop/Features/Send/SendModel.swift

@@ -96,6 +96,8 @@ final class SendModel: ObservableObject {
 				case .receiverHistoryChanged(let id), .approvalChanged(let id):
 					if id == self.state.selectedTransferId { self.refreshReceivers(id) }
 					self.refreshReceiverStatuses(for: id)
+				case .contactsChanged, .offersChanged:
+					break
 				}
 			}
 			.store(in: &cancellables)