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/Theme/VniDropTheme.swift

@@ -0,0 +1,56 @@
+import SwiftUI
+
+/// User-facing theme selection, mirrors `ThemeMode` in the Compose theme.
+enum ThemeMode: String, CaseIterable, Codable, Sendable {
+	case system
+	case light
+	case dark
+
+	/// SwiftUI color-scheme override (`nil` follows the system).
+	var preferredColorScheme: ColorScheme? {
+		switch self {
+		case .system: return nil
+		case .light: return .light
+		case .dark: return .dark
+		}
+	}
+}
+
+func resolveDarkTheme(_ mode: ThemeMode, systemDark: Bool) -> Bool {
+	switch mode {
+	case .system: return systemDark
+	case .light: return false
+	case .dark: return true
+	}
+}
+
+private struct VniDropColorsKey: EnvironmentKey {
+	static let defaultValue = VniDropColors.light
+}
+
+extension EnvironmentValues {
+	/// Semantic VniDrop tokens for the active theme. Read with
+	/// `@Environment(\.vniColors) private var colors`.
+	var vniColors: VniDropColors {
+		get { self[VniDropColorsKey.self] }
+		set { self[VniDropColorsKey.self] = newValue }
+	}
+}
+
+/// Provides the semantic token set matching the resolved light/dark theme to the
+/// whole subtree. Apply once near the app root.
+struct VniDropTheme: ViewModifier {
+	let isDark: Bool
+
+	func body(content: Content) -> some View {
+		content
+			.environment(\.vniColors, isDark ? .dark : .light)
+			.tint(VniDropColors.brandPurple)
+	}
+}
+
+extension View {
+	func vniDropTheme(isDark: Bool) -> some View {
+		modifier(VniDropTheme(isDark: isDark))
+	}
+}