feat: collapse subtasks from parent rows

frontend/src/lib/task-utils.ts

@@ -5,6 +5,7 @@ export type MinimalTask = {
   parent_id?: string | null
   completed?: boolean
   list_name?: string
+  subtasks?: MinimalTask[]
 }
 
 const escapeHtml = (value: string) =>
@@ -85,7 +86,10 @@ export function groupTaskTree<T extends MinimalTask>(tasks: T[]) {
     if (!task.parent_id) continue
     children.set(task.parent_id, [...(children.get(task.parent_id) ?? []), task])
   }
-  return tasks.filter((task) => !task.parent_id).map((task) => ({ task, subtasks: children.get(task.id) ?? [] }))
+  return tasks.filter((task) => !task.parent_id).map((task) => ({
+    task,
+    subtasks: children.get(task.id) ?? (task.subtasks as T[] | undefined) ?? [],
+  }))
 }
 
 export function moveItemWithinScope<T extends { id: string; parent_id?: string | null }>(items: T[], sourceId: string, targetId: string, placement: 'before' | 'after') {