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/Resources/Assets.xcassets/AppIcon.appiconset/Contents.json

@@ -0,0 +1,7 @@
+{
+  "images" : [
+    { "idiom" : "universal", "platform" : "ios", "size" : "1024x1024", "filename" : "app-icon.png" },
+    { "idiom" : "mac", "scale" : "2x", "size" : "512x512", "filename" : "app-icon.png" }
+  ],
+  "info" : { "author" : "xcode", "version" : 1 }
+}