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.

crates/uniffi-bindgen/src/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    uniffi::uniffi_bindgen_main()
+}