[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>

trx-client.toml.example

@@ -47,7 +47,3 @@ 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