feat(apple): send a transfer to a device from the share panel

Send to a device now sits alongside the QR code, NFC, and export actions,
since an offer is another way to deliver the same invitation. Picking a
device pushes the existing transfer rather than re-sharing the files.

The picker lists only devices holding a live grant, so nothing offered there
can fail on tap, and it distinguishes accepted from waiting for that device
to open the app.

Also fixes the deprecated SF Symbol and the two Sendable warnings introduced
with the contacts screen: the sections now talk to the model directly rather
than storing view callbacks that a Binding setter has to convert.

apple/VniDrop/Core/CoreGateway.swift

@@ -66,6 +66,11 @@ protocol CoreGateway: AnyObject {
 		transferName: String,
 		senderName: String
 	) async -> Result<ContactSendOutcome, Error>
+	/// Offer an existing share to a remembered device, alongside its QR code.
+	func offerTransferToContact(
+		transferId: UInt64,
+		endpointId: String
+	) async -> Result<ContactSendOutcome, Error>
 	/// Transfers this device is holding for contacts that were not running.
 	func heldOffers() async -> Result<[HeldOfferModel], Error>
 	/// Ask remembered devices whether they hold anything for this one.