When multiple sessions are active/settled simultaneously, survivors
(sessions the server omitted from the fresh page) were prepended as a
block in their old relative order from the previous $sessions array.
This caused recently-interacted sessions to appear below older ones.
Now survivors are sorted by last_active descending and merged into the
incoming array at the correct position using a two-pointer merge,
so the sidebar always reflects true recency.
Fixes#47203
liuhao1024
d029575407