Add the real FreeRDP3 RDP backend that loads libfreerdp3.dll at runtime
via syscall.NewLazyDLL (no CGO required). The Windows implementation
(freerdp_windows.go) calls FreeRDP3 functions for instance lifecycle,
settings configuration, input forwarding, and event handling. A stub
(freerdp_stub.go) compiles on non-Windows platforms and returns errors,
keeping macOS/Linux development and tests working. A factory function
(freerdp_factory.go) selects the right backend based on runtime.GOOS,
and app.go now uses NewProductionBackend instead of always using mock.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
