Added private notes for mentor requests, and a nudge to come prepared

Two small but real additions to the mentor request flow: before booking time with a mentor, there's now a short check-in confirming you know what you want to get out of the session — nothing heavy, just a moment to make sure you show up with a specific goal rather than "any advice welcome."

You can also now leave yourself private notes after a request — what you took away, what you're doing next, and an optional follow-up date to check back in. These are yours alone; the mentor never sees them.