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/VnidropCore/README.md

@@ -0,0 +1,25 @@
+# VnidropCore (Swift package)
+
+Swift bindings for the VniDrop Rust transfer core (`crates/vnidrop`), generated
+with UniFFI in library mode. The Rust crate is never modified for this — the
+Swift surface is produced from the compiled staticlib.
+
+## Regenerate
+
+From the repository root:
+
+```bash
+apple/scripts/build-core.sh release   # or: debug
+```
+
+This builds `libvnidrop.a` for `aarch64-apple-ios`, `aarch64-apple-ios-sim`, and
+`aarch64-apple-darwin`, generates `Sources/VnidropCore/Vnidrop.swift`, and
+assembles `vnidrop.xcframework`.
+
+## Generated / ignored artifacts
+
+- `vnidrop.xcframework/`
+- `Sources/VnidropCore/Vnidrop.swift`
+
+Both are gitignored. A clean checkout must run the build script before opening
+the Xcode project.