feat(apple): native SwiftUI app for iOS and macOS

Add a native SwiftUI VniDrop app (Send/Receive/Settings) talking to the
Rust core via generated UniFFI Swift bindings, plus the uniffi-bindgen
helper crate. iOS uses a TabView, macOS a NavigationSplitView sidebar.

apple/VniDrop/Platform/InvitationFile.swift

@@ -0,0 +1,21 @@
+import Foundation
+
+/// Builds a `.vnd` invitation filename from a transfer name, mirroring the iOS
+/// helper in `TransferShareActions.ios.kt`.
+func invitationFileName(_ transferName: String) -> String {
+	let trimmed = transferName.trimmingCharacters(in: .whitespacesAndNewlines)
+	let base = trimmed.isEmpty ? "invitation" : trimmed
+	let safe = base.components(separatedBy: CharacterSet.alphanumerics.inverted.subtracting(CharacterSet(charactersIn: "-_ ")))
+		.joined()
+		.replacingOccurrences(of: " ", with: "-")
+	let name = safe.isEmpty ? "invitation" : safe
+	return "\(name).\(vniDropInvitationExtension)"
+}
+
+/// Writes a temporary `.vnd` file for sharing/exporting.
+func writeTemporaryInvitation(ticket: String, transferName: String) throws -> URL {
+	let dir = FileManager.default.temporaryDirectory
+	let url = dir.appendingPathComponent(invitationFileName(transferName))
+	try ticket.write(to: url, atomically: true, encoding: .utf8)
+	return url
+}