ui: run every compaction summary through one path
Three callers needed the same thing - the automatic pre-encode pass, the
action in the context gauge, and re-running a summary from the block - and
each of them had grown its own copy of the request, the streaming and the
storing.
There is one path now: a summarization owns a node, streams into it, and
reports itself through a ledger the rest of the app reads (`isCompacting`,
which node belongs to which conversation). Re-running a summary is the same
call on an existing node, so the two entry points cannot drift apart again.
