fix(overview): add subtasks expand parameter to show task hierarchy

The overview pages (Done, Current, etc.) were not requesting subtask
relationship data from the API, causing task indentation and hierarchy
to be missing. This was inconsistent with project list views which
correctly request subtasks.

Add 'subtasks' to the expand parameter array alongside existing
comment_count and is_unread parameters. The backend API supports
multiple expand values, so this doesn't break existing functionality.

Fixes #2490

frontend/src/views/tasks/ShowTasks.vue

@@ -252,7 +252,7 @@ async function loadPendingTasks(from: Date|string, to: Date|string, filterId: nu
 		filter: 'done = false',
 		filter_include_nulls: props.showNulls,
 		s: '',
-		expand: ['comment_count', 'is_unread'],
+		expand: ['subtasks', 'comment_count', 'is_unread'],
 	}
 
 	if (!showAll.value) {