feat(ui): load a thread's replies when its row is expanded

Replies are fetched on expansion rather than with the query: walking the reply
tree of every thread in a 10k-thread result would cost more than the query and
almost none of it would be looked at.

hasChildren is what makes that lazy loading work, and its absence would have
shipped the feature unreachable. rowCount is 0 until the worker has walked the
thread, so a view left to infer the expander from rowCount alone draws none, the
user can never expand, and the replies are never requested. It answers from the
summary's totalCount before loading and from the children afterwards, so a
thread whose count included duplicates stops offering an expander that opens
onto nothing.

onThreadTreeLoaded reads the thread id from the reply rather than remembering it
from the request. Two expansions can be in flight at once, and pairing them by
order would attach one thread's replies to the other.

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