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/UI/Components/AdaptiveDrawer.swift

@@ -0,0 +1,66 @@
+import SwiftUI
+
+/// Presents modal content in a native sheet. On phones it uses medium/large
+/// detents with a grabber; on wider layouts the sheet is form-sized. Content is
+/// wrapped in a `NavigationStack` so it gets a native title bar + Close button.
+struct AdaptiveDrawer<DrawerContent: View>: ViewModifier {
+	@Binding var isPresented: Bool
+	let windowClass: WindowClass
+	let onDismiss: () -> Void
+	@ViewBuilder let drawerContent: () -> DrawerContent
+
+	func body(content: Content) -> some View {
+		content.sheet(
+			isPresented: Binding(get: { isPresented }, set: { if !$0 { onDismiss() } })
+		) {
+			SheetChrome(onClose: onDismiss) { drawerContent() }
+				.modifier(PhoneDetents(enabled: windowClass == .phone))
+		}
+	}
+}
+
+private struct PhoneDetents: ViewModifier {
+	let enabled: Bool
+	func body(content: Content) -> some View {
+		if enabled {
+			content
+				.presentationDetents([.medium, .large])
+				.presentationDragIndicator(.visible)
+		} else {
+			content.frame(minWidth: 460, minHeight: 480)
+		}
+	}
+}
+
+private struct SheetChrome<Content: View>: View {
+	let onClose: () -> Void
+	@ViewBuilder let content: () -> Content
+
+	var body: some View {
+		NavigationStack {
+			ScrollView { content().padding(.top, 4) }
+				.toolbar {
+					ToolbarItem(placement: .cancellationAction) {
+						Button(String(localized: "button_close"), action: onClose)
+					}
+				}
+				#if os(iOS)
+				.navigationBarTitleDisplayMode(.inline)
+				#endif
+		}
+	}
+}
+
+extension View {
+	func adaptiveDrawer<DrawerContent: View>(
+		isPresented: Binding<Bool>,
+		windowClass: WindowClass,
+		onDismiss: @escaping () -> Void,
+		@ViewBuilder content: @escaping () -> DrawerContent
+	) -> some View {
+		modifier(AdaptiveDrawer(
+			isPresented: isPresented, windowClass: windowClass,
+			onDismiss: onDismiss, drawerContent: content
+		))
+	}
+}