Rail grouping (按时间 / 按项目) does not persist across restarts #3060

Problem

viewMode (按时间 / 按项目) is useState<SessionViewMode>('conversation') in apps/desktop/src/renderer/app-shell.tsx. It resets to 按时间 on every restart.

This predates #3033, but that change promotes the control to permanent sticky chrome at the top of the rail — a full-width SegmentedControl that is always visible — so a preference silently reverting is now visible every launch rather than hidden in a dropdown.

The rail's collapsed state already persists through session-list-layout.ts (maka-chat-list-collapsed-v1); grouping should use the same seam.

Context

Found by a subagent during the adversarial review of #3033.