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/App/VniDropApp.swift

@@ -0,0 +1,40 @@
+import SwiftUI
+
+/// App entry point for iOS/iPadOS/macOS, ported from `iOSApp.swift` + `App.kt`.
+/// Opens `.vnd` invitations via `onOpenURL` and routes them to the receive flow.
+@main
+struct VniDropApp: App {
+	@StateObject private var externalInvitations = ExternalInvitationController()
+
+	var body: some Scene {
+		WindowGroup {
+			RootView(dependencies: makeAppDependencies(externalInvitations: externalInvitations))
+				.ignoresSafeArea()
+				.onOpenURL(perform: openInvitation)
+		}
+	}
+
+	/// Reads a `.vnd` invitation document under a security scope, enforcing the
+	/// 64 KiB / strict-UTF-8 rules from `ContentView.swift`.
+	private func openInvitation(_ url: URL) {
+		guard url.pathExtension.caseInsensitiveCompare(vniDropInvitationExtension) == .orderedSame else {
+			externalInvitations.reportOpenFailure(message: "This is not a VniDrop invitation")
+			return
+		}
+		let started = url.startAccessingSecurityScopedResource()
+		defer { if started { url.stopAccessingSecurityScopedResource() } }
+		do {
+			let values = try url.resourceValues(forKeys: [.fileSizeKey])
+			if let size = values.fileSize, size > maxVniDropInvitationBytes {
+				throw InvitationError.tooLarge
+			}
+			let data = try Data(contentsOf: url, options: .mappedIfSafe)
+			let raw = try decodeInvitationBytes(data)
+			externalInvitations.openInvitation(raw: raw)
+		} catch {
+			externalInvitations.reportOpenFailure(
+				message: (error as? LocalizedError)?.errorDescription ?? "The invitation could not be opened"
+			)
+		}
+	}
+}