wantsSnapcompact is true for both the default strategy and an explicit
/compact snapcompact mode override. The prior fix downgraded both to LLM
compaction, which silently shipped the transcript to a provider for users
who deliberately requested the local-only no-LLM archive path. Only the
default-configured strategy now falls back; explicit snapcompact keeps
failing locally. Added a regression test for the explicit path.
Fixes#5064
Manual /compact with the snapcompact strategy hard-threw when the active
model lacked image input, unlike the auto-compaction path which downgrades
to LLM-backed compaction. Clear snapcompactReady instead of throwing so the
flow falls through to #compactWithFallbackModel (active text-only model
tried first for text->text summarization).
Fixes#5064
roboomp