* fix(web): keep dockview group alive on task switch to stop layout collapse
On a fast-path env switch, the incoming session chat was added only after
removeEphemeralPanels closed the outgoing chat. When that chat was the last
surviving panel in its group (e.g. the default layout's chat column), removing
it first emptied — and so destroyed — the group. The captured group id then no
longer existed, and post-#1165 the old `referenceGroup: "sidebar"` fallback is
dead, so addPanel ran with an undefined position: dockview dropped the incoming
chat into the active (terminal) group and collapsed the grid root to a vertical
stack. The corrupted layout was then persisted, so it survived reloads.
Add the incoming session panel before removing the outgoing one, anchored at the
outgoing panel's current index, so the group stays alive throughout the swap.
Removing the panels ahead of the insert point shifts the new panel into the same
final slot, so computeSurvivingIndex is no longer needed.
* docs(web): note the raw-index equivalence assumption in env switch
José Almeida
46f20e0dfe