Remote desktop from Windows: sign in as steamos, and say when xrdp isn't there

A Windows user reported "RDP not working". Frame Control ran `mstsc /v:HOST`,
which offers the Windows account; the Frame's xrdp (TLS, no NLA) only accepts
steamos with the Developer Mode password. The app also said "Opened Remote
Desktop" without checking that anything answered on port 3389.

- open_rdp checks port 3389 first and explains how to turn xrdp on
- On Windows, launch mstsc with a .rdp file naming user steamos (CRLF)
- Every platform's message says to sign in as steamos with the Developer Mode password
- The server no longer logs a page closing mid-reply (WinError 10053 on
  Windows) as a 500 with an error diagnostic

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/frame-control.md

@@ -88,8 +88,12 @@ counts them while they run.
   name your networks, and switch headsets. See [devices.md](devices.md).
 - **One-click tools**: SSH or SFTP in a terminal window, Steam Link, and remote
   desktop (Windows App on macOS, Remote Desktop on Windows, Remmina or FreeRDP on
-  Linux). Sleep, restart and shut down open a terminal window because SteamOS
-  asks for the sudo password over SSH.
+  Linux). Remote desktop first checks that the Frame's xrdp answers on port
+  3389 (Developer Mode turns it on). On Windows it opens a connection file for
+  user `steamos`, because `mstsc /v:` alone offers your Windows account, which
+  xrdp turns away. Sign in with the Developer Mode password. Sleep, restart and
+  shut down open a terminal window because SteamOS asks for the sudo password
+  over SSH.
 
 ## How it works