Welcome experience for new signups + public concept explainer

New signups now get a short first-run tour on their first visit to the app, walking through how memsprout thinks: everything is a Memory, Spaces decide who sees it, and Topics group related Memories — plus how agents read and write the same Memories over MCP. The public site now explains this same model in a new "The model" section with a Space → Topic → Memory diagram, right after "How it works".