Hide sidebar nav links to pages absent from the core build #3

In the open-core build, the app sidebar (apps/web/src/components/app/Sidebar.tsx) still renders nav rows for Graph, Meetings, and Admin — but those pages are enterprise-only and aren't present here, so the links dead-end.

Task: gate those nav rows so they don't render when the corresponding route/feature isn't available in this build. Keep it data-driven, not a giant hardcoded conditional.

Files: apps/web/src/components/app/Sidebar.tsx

Acceptance criteria

• The core sidebar shows no dead links (no Graph/Meetings/Admin rows that 404).
• Present nav items are unchanged; no new console errors.

Size: S