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/Buttons.swift

@@ -0,0 +1,50 @@
+import SwiftUI
+
+/// Native SwiftUI button styles. Purple accent comes from the app-wide `.tint`.
+
+/// Full-width filled button (`.borderedProminent`). Apply `.fixedSize()` at the
+/// call site to shrink it to its content.
+struct PrimaryButton: View {
+	let title: String
+	let action: () -> Void
+	var enabled: Bool = true
+
+	var body: some View {
+		Button(action: action) {
+			Text(title).frame(maxWidth: .infinity).frame(minHeight: 22)
+		}
+		.buttonStyle(.borderedProminent)
+		.controlSize(.large)
+		.disabled(!enabled)
+	}
+}
+
+/// Full-width bordered (secondary) button.
+struct SecondaryButton: View {
+	let title: String
+	let action: () -> Void
+	var enabled: Bool = true
+
+	var body: some View {
+		Button(action: action) {
+			Text(title).frame(maxWidth: .infinity).frame(minHeight: 22)
+		}
+		.buttonStyle(.bordered)
+		.controlSize(.large)
+		.disabled(!enabled)
+	}
+}
+
+/// Borderless tinted text button.
+struct QuietButton: View {
+	let title: String
+	let action: () -> Void
+	var enabled: Bool = true
+
+	var body: some View {
+		Button(title, action: action)
+			.buttonStyle(.borderless)
+			.disabled(!enabled)
+	}
+}
+