Lists: parent/child hierarchy and breadcrumbs #64

Product behaviour (/help/lists)

Lists form a tree via a parent-child relationship: create a parent list (e.g. "Projects"), then optionally set a parent when creating a new list (local or GitHub-backed). Breadcrumbs navigate up and down the hierarchy. For GitHub-backed lists the parent is changed via Edit after creation.

GET /api/users/{username}/lists/{id} explicitly returns "public list metadata and ancestor chain for breadcrumbs".

Acceptance criteria

• parentId settable on create and on edit.
• Breadcrumb bar on the list detail view, each segment clickable.
• Child lists are discoverable from the parent.
• A list cannot be made its own ancestor (cycle guard).