[Enhancement] Role change flow with confirmation and audit note

[techifydev1]

Allow admins to change a user's role (student <-> mentor <-> admin).

• select control in the Actions tab of user detail
• confirmation dialog stating consequences in plain language (e.g. mentor gains creator tools)
• require an optional reason note that gets sent with the request
• optimistic UI off: wait for the API response, then toast success/failure

Changing someone TO admin should be extra guarded: type-the-email confirmation, no bulk path may ever include this action.