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/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "uniffi-bindgen"
+version = "0.1.0"
+edition = "2021"
+license = "Apache-2.0"
+publish = false
+
+# Standalone UniFFI bindings generator used to produce the Swift bindings for the
+# Apple app. It intentionally pins the same UniFFI version as `crates/vnidrop` so
+# generated bindings never drift from the scaffolding compiled into the library.
+[[bin]]
+name = "uniffi-bindgen"
+path = "src/main.rs"
+
+[dependencies]
+uniffi = { version = "=0.29.4", features = ["cli"] }

crates/uniffi-bindgen/src/main.rs

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