- Move the expand chevron into a gutter left of the folder icon so folders
with and without subfolders line up on their icon; labels share the column.
- Add "Mark all as read, incl. subfolders" to the folder context menu, with
the affected count and a per-folder fallback for servers without filter
operators.
- Re-measure the virtualised message list when row height changes.
- Mock: seed unread mail in a subfolder.