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.toml

@@ -15,6 +15,9 @@ serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 url = "2"
 
+[target.'cfg(target_os = "linux")'.dependencies]
+gtk = { version = "0.18", features = ["v3_24"] }
+
 [features]
 default = ["custom-protocol"]
 custom-protocol = ["tauri/custom-protocol"]