Add superuser admin interface for managing organizations and users:
- Admin layout with access control (redirects non-superusers)
- Admin dashboard with org/user counts and quick actions
- Org management: list, create, view/edit details, manage sites
- User management: list, view details, toggle superuser, confirm email
- SuperuserBadge component for consistent superuser indication
- Sidebar shows admin link (shield icon) for superusers only
- Centralized date formatting utility at $lib/utils/format-date.ts
- Test plan documentation at docs/test-plans/admin.md
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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