Remove CSD titlebar buttons, fix tray toggle and quit

- Replace GTK header bar with buttonless version to work around
  upstream bug where CSD buttons become non-interactive after
  hide()/show() (tauri-apps/tauri#11856, tauri-apps/tao#1046)
- Tray menu now toggles between "Hide Claude" / "Show Claude"
- Left-click tray icon toggles window visibility
- Fix quit from tray (use std::process::exit to bypass keep-alive)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src-tauri/Cargo.lock

@@ -451,6 +451,7 @@ dependencies = [
 name = "claude-app"
 version = "1.0.0"
 dependencies = [
+ "gtk",
  "serde",
  "serde_json",
  "tauri",