[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>

trx-client.toml.example

@@ -47,3 +47,7 @@ listen = "127.0.0.1"
 port = 0
 # List of accepted bearer tokens (empty = no auth)
 # auth.tokens = ["example-token"]
+
+[frontends.appkit]
+# Enable AppKit GUI frontend (macOS only, requires appkit-frontend feature)
+enabled = false