Seed sandboxes in setup_demo so the sandbox UX is demonstrable out of the box

Problem

Lightning.SetupUtils.setup_demo/1 seeds two demo projects (OpenHIE and DHIS2) but no sandboxes. Anyone walking through the sandbox feature on a fresh demo instance has to provision them by hand to see the UI in a realistic state.

Suggested fix

Have setup_demo also provision three sandboxes after the projects are created:

• openhie-dev under OpenHIE (created by Amy as admin; all four demo users inherit because they're all on OpenHIE).
• dhis2-dev under DHIS2 (created by Amy as owner; only Amy is on DHIS2, so the sandbox starts with just her).
• dhis2-feature-x nested under dhis2-dev (demonstrates the nesting feature, the breadcrumb truncation, and the picker collapsing hidden ancestors).

That gives a demo state where Sizwe naturally cannot see anything under DHIS2 (the visibility filter in action) while still being able to demo OpenHIE and its sandbox. Useful for clicking through the sandbox docs without any extra setup.