ActiveThreadSync was reacting to aui's mainThreadId === null on mount
(aui hasn't booted yet) by calling setActiveThreadId(null), which
wiped the just-restored persisted draft id from localStorage and
emptied the doc panel for the user's thread. The previous fix only
covered the 'aui minted a different LOCALID' branch; it missed the
'mainThreadId is null while aui boots' branch.
Treat a null mainThreadId as a no-op for the sync. Explicit clears
(new chat, sidebar delete) keep going through setActiveThreadId(null)
directly, so this guard doesn't trap stale state — it just gives the
persisted draft id a chance to survive until aui finishes booting.
3d3a00c2be