Add discoverable rooms to desktop lobby

desktop-client/README.md

@@ -8,6 +8,18 @@ Preferences are stored locally using Qt's platform settings store. File dialogs
 for both creating and joining rooms start in the remembered folder. The server
 address remains fixed. Detached chat opens docked on the next launch.
 
+## Active sessions
+
+The lobby checks for listed rooms at startup and every 15 seconds while open.
+Select a session and click **Choose file & join**, or double-click it, to use the
+existing file verification flow. Enter your display name first. **Refresh** checks
+immediately; joining by code remains available when discovery fails.
+
+New rooms are listed by default. Clear **List room in active sessions** before
+creating a room to keep it out of the list; anyone with its code can still join.
+Only rooms with connected participants appear. Discovery requires the updated
+server's `/api/rooms` endpoint; older clients also create listed rooms by default.
+
 ## Chat
 
 Chat follows new messages while you are near the bottom. Scroll up to read older