[gui] synchronization sometimes breaks

[dskvr]

If a tab goes idle while live sync is running, then the lastCompleteSync timestamp will get updated onActive without any fresh data due to a browser-(terminated || paused [browser dependent]) worker.

Upon the tab becoming active again, the worker might resume (browser dependent behavior), and the live sync will continue, which includes clearing "stale" checks from the stores. However, since it was idle, almost all the checks from the stores will be removed, and wihtout a refresh or wipe, it would take the duration of the selected monitor's frequencies to repopulate the stores.

Need idle detection that plays nicely with tab leader management to...

• pause livesync if enabled
• on active, run full sync (fetch) and resume livesync if it was enabled; should reconcile dataset
• add condition to removeStaleChecks so that it does not run until dataset is repopulated (to avoid clearing stores and starting from zero)