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/Feedback/SnackbarHost.swift

@@ -0,0 +1,79 @@
+import SwiftUI
+
+/// Bottom toast host driven by `UiMessageController`, ported from
+/// `ui/feedback/VniDropSnackbarHost.kt`. Tone drives the accent color; errors get
+/// a longer display duration.
+struct SnackbarHost: View {
+	@ObservedObject var controller: UiMessageController
+	@State private var dismissTask: Task<Void, Never>?
+
+	var body: some View {
+		VStack {
+			Spacer()
+			if let message = controller.current {
+				content(for: message)
+					.frame(maxWidth: 520)
+					.background(.regularMaterial, in: RoundedRectangle(cornerRadius: 14))
+					.overlay(RoundedRectangle(cornerRadius: 14).stroke(.gray.opacity(0.25), lineWidth: 0.5))
+					.shadow(color: .black.opacity(0.15), radius: 8, y: 2)
+					.padding(.horizontal, 16)
+					.padding(.bottom, 8)
+					.transition(.move(edge: .bottom).combined(with: .opacity))
+					.id(message.id)
+					.onAppear { scheduleDismiss(for: message) }
+			}
+		}
+		.animation(.easeInOut(duration: 0.2), value: controller.current?.id)
+	}
+
+	@ViewBuilder
+	private func content(for message: UiMessage) -> some View {
+		let accent: Color = {
+			switch message.tone {
+			case .info: return VniDropColors.brandPurple
+			case .success: return .green
+			case .warning: return .orange
+			case .error: return .red
+			}
+		}()
+		HStack(alignment: .center, spacing: 8) {
+			Circle().fill(accent).frame(width: 8, height: 8)
+			Text(message.text.resolved())
+				.font(.subheadline)
+				.frame(maxWidth: .infinity, alignment: .leading)
+				.padding(.vertical, 10)
+			if let actionLabel = message.actionLabel {
+				Button(action: {
+					message.onAction?()
+					dismiss()
+				}) {
+					Text(actionLabel.resolved()).fontWeight(.semibold)
+				}
+				.buttonStyle(.borderless)
+			}
+			Button(action: dismiss) {
+				Image(systemName: "xmark")
+					.font(.footnote.weight(.semibold))
+					.foregroundStyle(.secondary)
+					.frame(width: 36, height: 36)
+			}
+			.buttonStyle(.plain)
+		}
+		.padding(.leading, 16)
+		.padding(.trailing, 4)
+	}
+
+	private func scheduleDismiss(for message: UiMessage) {
+		dismissTask?.cancel()
+		let seconds: UInt64 = message.tone == .error ? 6 : 4
+		dismissTask = Task {
+			try? await Task.sleep(nanoseconds: seconds * 1_000_000_000)
+			if !Task.isCancelled { controller.advance() }
+		}
+	}
+
+	private func dismiss() {
+		dismissTask?.cancel()
+		controller.advance()
+	}
+}