Server: remove activityAt computation from workspace descriptors (always
sends null for backwards compat) and deduplicate emissions in
emitWorkspaceUpdatesForWorkspaceIds using fast-deep-equal — workspace
updates are now only sent when the descriptor actually changed.
Client: remove activityAt from WorkspaceDescriptor, sidebar entry types,
and baseline sort comparators. Workspace/project ordering is fully
handled by the persisted sidebar order store.