Clicking New Session reuses the workspace's existing blank session when one
exists; reuse only opens it, so its updatedAt (host: max(createdAt,
lastPromptAt)) never advances and the row kept its old creation-time
position in the sidebar. nextSessionOrderAccount now pins the currently
selected blank session to the front of its account in both Manual and Last
updated modes; non-blank navigation stays untouched.
Fixes#2788