New /admin route (frontend/src/admin/), gated by a separate admin
session (scrypt + timingSafeEqual against ADMIN_PASSWORD, no plaintext
storage, no new dependency). Covers cohort CRUD + roster + curriculum
assignment, and student CRUD + cohort assignment + environment/progress/
attempt views, all under /api/admin/* on the Platform API.
main.jsx now does a three-way path split (lab / admin / platform) so the
existing, unmodified Lab UI, the student dashboard, and the new admin
portal all ship in one Vite bundle without a router dependency.
