Devices: one Frame Control server per user

Two servers each connected, reconnected and edited the headsets on their own,
and several review findings were ways one could move the other's install to a
different headset. A lock file in the data folder now refuses a second server
with a plain message; FRAME_CONTROL_DATA_DIR still gives a separate one.

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

docs/devices.md

@@ -125,6 +125,14 @@ the key it shows, as Set Up Connection does. After reinstalling SteamOS the
 headset has a new key; **Forget identity** on the Devices tab lets the next
 connection save the new one.
 
+## One server at a time
+
+Only one Frame Control server runs per user (a lock file, `server.lock`, in the app's
+data folder). Two would each connect, reconnect and edit the headsets on their own, and
+one could move the other's install to a different headset. A second one, say
+`scripts/frame-ui.sh` while the app is open, exits with "Frame Control is already
+running". `FRAME_CONTROL_DATA_DIR` gives a separate one, with its own headsets.
+
 ## API
 
 All under the usual `/api/` guards (loopback `Host`, `X-Frame-UI` header).