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.

internal/desktop/process_unix.go

@@ -0,0 +1,17 @@
+//go:build !windows
+
+package desktop
+
+import (
+	"errors"
+	"syscall"
+)
+
+func processAlive(pid int) bool {
+	err := syscall.Kill(pid, 0)
+	return err == nil || errors.Is(err, syscall.EPERM)
+}
+
+func terminateProcess(pid int) {
+	_ = syscall.Kill(pid, syscall.SIGTERM)
+}