[chore](trx-client): remove appkit frontend support

Remove macOS AppKit frontend (trx-frontend-appkit) and related code:
- Delete appkit crate directory
- Remove appkit dependency and feature from Cargo.toml
- Remove appkit imports, main thread handling, and config from main.rs
- Remove AppKit config struct from config.rs
- Remove appkit section from example config

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

Cargo.toml

@@ -13,7 +13,6 @@ members = [
     "src/trx-client/trx-frontend",
     "src/trx-client/trx-frontend/trx-frontend-http",
     "src/trx-client/trx-frontend/trx-frontend-http-json",
-    "src/trx-client/trx-frontend/trx-frontend-appkit",
     "src/trx-client/trx-frontend/trx-frontend-rigctl",
     "src/trx-core",
 ]