feat(jetbrains): toggle the worktree session list

Add a toggle left of the worktree editor toolbar that shows or hides the
session list. The choice is persisted per worktree through the backend
instead of a single global property, so each worktree reopens the way the
user left it. While the list is hidden the toggle carries the session
count, or the activity icon of a background session that needs the user,
so a pending question stays visible when collapsed.

Give the toolbar strip standard horizontal-toolbar padding on its left,
top and bottom, keeping the right edge flush so the divider still sits
against the header content, and separate the toggle from the toolbar
actions with a vertical separator. The toggle is centred at its own
height rather than tracking the strip, so its hover box matches a regular
toolbar button instead of running edge to edge.