Wire up the existing parent_id data model into a full subtask UI:
- Backend: create_task accepts parent_id, cascade delete/toggle for subtasks
- Store: subtask-aware derived state (root tasks only in pending/completed),
childrenMap for O(1) subtask lookup, createTask accepts parentId
- TaskItem: depth-based indentation, subtask completion counter badge
- TasksScreen: expand/collapse chevron for parent tasks, indented subtask
rendering with left border, subtasks shown under completed parents
- TaskDetailView: "Add subtask" inline input, subtask list with toggleable
checkboxes, parent task breadcrumb for subtasks
https://claude.ai/code/session_01XWcSekaCAJ7qnr4hN3tgL8
45b7cc57de