feat(ui): open list items counter on Listen, none on Workspace

New stat listItemsOpenCount: unchecked items in all lists the user sees
(own, shared directly or through a group), shown as accent counter on
Listen in the header and the mobile bottom bar. Workspace loses its
counter. Stats refresh on list events and after deleting an item.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/lib/types.ts

@@ -490,6 +490,7 @@ export interface DashboardStats {
   tasksDoneCount: number;
   docsCount: number;
   docsTodosOpenCount?: number;
+  listItemsOpenCount?: number;
   listsCount: number;
   discardedCount: number;
 }