Mark Budget setup as done

What your assistant is told

Mark the user's Loki Budget setup as done, so the app stops showing its "finish setting up" card on Home. Call this at the end of the first-run setup chat (the one the app opens with "When we're finished, mark my setup as done."), once the user has confirmed their setup - accounts, categories, budgets - looks right. Don't call it mid-setup or on your own initiative in a later chat. Takes no arguments. Idempotent: the first call records the time, later calls keep it. Returns a confirmation you can pass on to the user, and when setup was marked done.