[US-004] Store admin separate onboarding flow

[leimo2011]

Epic: EP-01 — User Authentication & Profiles \nPriority: Must Have (M) \nStory Points: 5 \nSprint: Sprint 1\n\n### User Story\nAs a store admin, I want a separate onboarding flow so my dashboard differs from end users.\n\n### Acceptance Criteria\n- [ ] Store admin registration is a distinct flow from end user\n- [ ] Admin dashboard redirected after login (not end user home)\n- [ ] Role ADMIN assigned in database\n- [ ] Store admin prompted to complete store profile after first login\n- [ ] Super admin can approve/reject store admin registrations\n\n### Technical Notes\n- Role-based routing in React (admin portal)\n- POST /api/auth/admin/register/