Support sub-items inside a collection

## Goal

Parent-child rows within a single collection. Useful for tasks with subtasks, projects with deliverables, that kind of thing. The parent row can show its children inline, and children know about their parent.

## Scope

* A parent relationship between rows in the same collection, stored as row meta.
* Render nested rows in table and list views: a disclosure triangle on the parent, children indented underneath.
* Add a child from the parent row's menu, or convert an existing row into a sub-item.
* The row detail view shows direct children.

## Out of scope

* Cross-collection parent-child.
* Multi-level grouping that mixes sub-items with group-by (decide later).
* Auto-rolled-up status from children (its own thing).

## Acceptance criteria

* You can mark a row as a child of another row in the same collection.
* The parent shows its children inline in table and list.
* Children appear in the detail view of their parent.
* Reparenting works from the row's menu.