Update email styling to memsprout design system

Auth emails (sign-in, signup confirmation, password reset, invitation, email change) now use the memsprout calm-infrastructure aesthetic: cool slate-toned colors, system-ui body font, SF Mono signal layer, and a clean white card on a light cool-gray background — replacing the retired editorial-broadsheet palette. Password reset and email-change emails no longer show a 6-digit OTP code, which was surfaced by accident and is not useful outside of MCP OAuth flows. All email subjects updated from "Vello" to "memsprout".