Closes#3106. Adds /api/session/title/regenerate endpoint + session-action
menu item. Preserves chronology (touch_updated_at=False), guards read-only
and imported sessions, syncs to state.db when Insights sync enabled.
Maintainer refinement (Opus SHOULD-FIX): scope the is_imported guard to the
regenerate action only instead of broadening the shared _isReadOnlySession()
helper, which also gates rename/pin/archive/move/fork. Matches the backend
403 guard. Test updated to assert the scoped shape.