Ports the legacy Next settings page (profile/appearance/notifications/
providers tabs) and admin page (user management + system health) into
spa/pages as faithful ports, replaces the /settings and /admin route
placeholders, and adds a route-level AdminGuard mirroring the legacy
AdminRoleGuard semantics (unauthenticated -> /login, non-admin -> /).
Legacy app/ tree untouched; removal stays in P5 per the increment map.