Remote desktop: connect FreeRDP only while a VNC viewer is connected

vnc-bridge.sh kept FreeRDP connected to krdpserver from the moment remote desktop
started, so krdp captured and H.264-encoded every KWin redraw in software (openh264)
with nobody watching: krdpserver 55-78% of a core, xfreerdp 16-27%, Xvnc 6-11%, with 0
clients on :5900. krdp 6.7 creates its screencast session per RDP connection and drops
it when the connection closes, so krdpserver itself idles without one and stays up.

The bridge now counts established connections to Xvnc's port with ss, starts FreeRDP
when a viewer appears (the desktop shows about 3 s later; the VNC screen is black until
then) and stops it 45 s after the last one leaves (VNC_IDLE_SEC). Xvnc has no client
hook, so its log output, which it writes for every connection, wakes the bridge early;
otherwise it looks every 5 s while idle (0.1% of a core measured, against 0.9% for ss
once a second) and every second while FreeRDP runs. The layout check runs only while
FreeRDP runs.

While a viewer is connected the bridge sends "watch 15" to ft-screens (@ft_screens) at
once and every 5 s, so screens at a reduced frame rate (out of view, headset on a
stand) stream at full rate; it lapses by itself if the bridge dies, and an ft-screens
without the command just answers an error. The window search after starting FreeRDP
now ends when FreeRDP exits instead of polling for 30 s.

krdp on 127.0.0.1 with a fresh password, VNC on the tailnet address with VncAuth, and
remote-ctl.sh start/stop (pause and resume) are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/design.md

@@ -177,6 +177,8 @@ The private runtime directory also moves the session's document portal to `$XDG_
 
 A podman container's monitor process (conmon) stays in the cgroup of whatever started the container, and `distrobox enter` starts it on demand. When a Frametop service happened to start the `dev` container, stopping that service stopped the container and everything in it, including the desktop's compositor. `scripts/container-up.sh` starts the container in a systemd scope of its own before anything enters it. It then waits for distrobox-init to log `container_setup_done`, as `distrobox enter` does only for containers it starts itself. A new container's first start takes a minute or more (it installs distrobox's dependencies and sets up passwordless sudo), and an install that entered right away met a sudo password prompt with no terminal to answer it ([#9](https://github.com/DeeJanuz/frametop/issues/9)).
 
+Remote desktop is a chain (krdp, then FreeRDP inside Xvnc) because nothing on SteamOS serves KWin over VNC directly. Kept connected all the time, it cost about a core with nobody watching: krdpserver 55 to 78% (it encodes H.264 in software with openh264: VA-API finds no driver for the Frame's GPU in the container), FreeRDP 16 to 27%, Xvnc 6 to 11%. krdp creates its screencast session per RDP connection and drops it when the connection closes (`SessionController::onNewConnection` in krdp 6.7), so an idle krdpserver costs nothing and can stay up; only the RDP connection has to go. The bridge connects FreeRDP when a VNC client appears and disconnects 45 seconds after the last one leaves. Xvnc has no hook for its clients, so the bridge counts established connections to its port with `ss`, woken early by Xvnc's log output; looking with `ss` once a second cost about 0.9% of a core in bash, against about 0.1% this way. `Xvnc -inetd` from a systemd socket would start a server per connection and lose sharing between viewers.
+
 Program names stay within 15 characters, because Linux truncates process names there and the scripts find programs with `pgrep -x` and `pkill -x`. That's why the prefix is `ft-`.
 
 ## Displays off on a stand