Add a cross-platform desktop app (Fyne)

The tunnel client now has a desktop UI over the same internal/client
service: status with live connection state, routes with add/remove/copy,
live logs, settings, profile import, and a start/stop button that runs
the bridge in-process. It uses the existing ~/.config/tunnel/config.json
and shows a warning banner with a stop button when the CLI service is
already running and claiming the routes.

cmd/tunnel-desktop/main.go

@@ -0,0 +1,11 @@
+package main
+
+import (
+	"gitea.tzgyn.dev/TZGyn/tunnel/internal/desktop"
+)
+
+var version = "dev"
+
+func main() {
+	desktop.Run(version)
+}