UsageStats/ModelUsageStat/ConversationUsageStat need explicit
nonisolated inits under this project's SWIFT_DEFAULT_ACTOR_ISOLATION =
MainActor setting, same as Message/Conversation. deleteFolder also
discarded FolderRecord.deleteOne's Bool result implicitly via the
write closure's return value.
Extends the existing Stats sheet (⌘⇧S) with a Session/All-Time segmented
picker. All-Time aggregates tokens, cost, and message counts across every
saved conversation, broken down by model and by conversation, using the
modelId already stored per message — no schema change needed.
rune