fix(webapp): stop session durations climbing forever when no run is live

Sessions kept an ever-growing wall-clock duration because the cell ticked for
any open session, even when its run had finished long ago. The duration now
ticks only while a run is genuinely executing; otherwise it freezes at the last
run's completion (or shows a dash if it never ran). Session status stays the
existing filterable ACTIVE/CLOSED/EXPIRED set, so there is nothing new to
filter. This drops the earlier display-only IDLE status, which was not
filterable.

Author: D-K-P