User create page:
- When role=admin, show super admin toggle
- If not super admin, show platform multi-select
- Admin users created via /api/v1/admin/admin-users endpoint
- Vendor users created via existing /admin/users endpoint
Vendor create page:
- Add platform selection section
- Vendors can be assigned to multiple platforms on creation
- Update VendorCreate schema to accept platform_ids
- Update AdminService.create_vendor() to create VendorPlatform records
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>