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_windows.go

@@ -0,0 +1,15 @@
+//go:build windows
+
+package desktop
+
+import "os"
+
+// The CLI background daemon is not supported on Windows; the desktop app
+// runs the service in-process, so there is no CLI pid to show or stop.
+func processAlive(int) bool { return false }
+
+func terminateProcess(pid int) {
+	if process, err := os.FindProcess(pid); err == nil {
+		_ = process.Kill()
+	}
+}