Add the Fyne desktop app (M5)

A cross-platform desktop UI over the same client service: status,
routes with add/remove/copy/open, live logs, settings, official-profile
import, and start/stop. It uses the existing JSON config and offers to
stop a running CLI daemon so the two do not fight over routes. The app
can autostart the service, refreshes status/config every few seconds,
and builds as a macOS .app via scripts/build-desktop-app.sh.

.gitignore

@@ -1,5 +1,10 @@
 /tunneld
+/tunnel
+/tunnel-desktop
+/Tunnel.app
 cmd/tunneld/tunneld
+cmd/tunnel/tunnel
+cmd/tunnel-desktop/tunnel-desktop
 .local/
 .DS_Store
 web/node_modules/