[feat](trx-frontend-appkit): add native macOS AppKit frontend

Add a new trx-frontend-appkit crate using objc2 + AppKit as a
replacement for the removed Qt/QML frontend. The frontend provides
the same feature set: frequency/mode/band display, PTT/power/VFO/lock
controls, signal/TX metering, and frequency/mode/TX-limit input.

Architecture splits platform-agnostic model (model.rs) from AppKit
UI (ui.rs) to facilitate future UIKit porting. State flows from the
async tokio watcher via std::sync::mpsc to the AppKit main thread;
button actions flow back through a channel to stay on the UI thread.

Feature-gated behind `appkit-frontend` cargo feature.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

README.md

@@ -17,6 +17,7 @@ The rig task is now driven by the controller components (state machine, handlers
 
 - HTTP status/control frontend (`trx-frontend-http`)
 - JSON TCP control frontend (`trx-frontend-http-json`)
+- AppKit GUI frontend (`trx-frontend-appkit`, macOS only, optional via `appkit-frontend` feature)
 - rigctl-compatible TCP frontend (`trx-frontend-rigctl`, listens on 127.0.0.1:4532)
 
 ## Plugin discovery