User-facing self-suggestion surface (is-this-you link-to-existing)

Fast-follow from PLUTO-617. Net-new user-facing self-suggestion surface: when a just-logged-in user has NO exact-email auto-link but a similarity candidate (name/DNI/legajo) exists, show an 'is this you? iniciá sesión / vinculá tu cuenta' suggestion so the USER can initiate the link-to-existing (routed to admin confirm), instead of only surfacing candidates on the admin solicitud-detail side. 617 v1 covers the admin-side confirm-link (admin CAS attach) fully; this is the reciprocal user-facing entry point. Reuses 617 resolver + attach infra.