fix(sync): close the server websocket once background data sync finishes #2212

The Exchange screen no longer disconnects the archive server websocket on blur while data sync is still in progress (see the linked pull request), so an upload can finish in the background. After that nothing on the phone closes the socket: core's $sync.stop() and the autostop only clear wantsToSyncData, and disconnectServers() has no caller outside the Exchange screen. The socket stays open, idle on core's keepalive, until the user next visits and leaves the screen or the process dies. This mirrors how local peer sync is left running off-screen, but it is a change in how long the server connection lives.

The connection needs an owner that outlives the screen, for example a hook in the active-project provider that calls projectApi.$sync.disconnectServers() when the sync state reaches complete-full or data sync is turned off (by the autostop or an explicit stop). The Exchange screen's own blur handling would then only need to cover the "sync not enabled" case.