detect idle sessions

[oliver-sanders]

(Previously discussed but not documented)

To help avoid idle resource it would be helpful if the Cylc UI would disconnect its websockets when no longer in use.

This could be implemented via an activity timeout, e.g:

• If no user activity (e.g. click / mouse movement) is detected within a configured period.
• We cut the websocket connection (i.e. disconnect and disable to reconnection interval).
• And put an overlay over the page saying something along the lines of "session suspended, click to resume".
• If the user clicks to resume, the websocket is reconnected and the session continues as normal (i.e. re-enable the reconnection interval).

This would help to clean up necessary websocket connections, e.g. from background browser tabs left open overnight.

Cleaning up these connections will help us to clean up Cylc UI Server connections which is helpful for releasing resource and helping servers stay up to date with Cylc versions.

[MetRonnie]

Could use https://vueuse.org/core/useIdle/