Loading thread item

A placeholder for a thread list item being fetched from network

Skeleton placeholder for a single row in a thread/message list. Renders as a 2-column (`auto 1fr`) by 2-row grid: a loading avatar in the left column, vertically centred and spanning both rows; a half-width loading line in the top-right cell; and a full-width loading line in the bottom-right cell. Use it while thread data is loading to reserve layout and hint at the shape of the incoming content. For the card-wrapped variant (avatar plus a configurable number of text rows inside an `fl-card`), use `fl-loading-avatar-row-card` instead.