The conversation view, removed

Selecting a thread root rendered the whole conversation here: earlier
messages as unexpandable stubs, the last two expanded. Removed at the
user's request on 2026-08-14, in favour of every row rendering exactly
one message.

Tagged for retrieval. The render path is in MainWindow::onThreadLoaded
and the fallback that reached it is at the end of onThreadSelected, both
as they stood in this commit's parent. NotmuchWorker::loadThread still
exists on master with no UI caller.

See backlog item 66 in 2026-08-03-post-0.1.0-usability-closed.md.
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