Add src/admin/ui/SidebarNav.tsx: drawer content component using MUI
List/ListItemButton with NavLink polymorphism. Active item uses the
`selected` prop for the white "pill" styling (not NavLink active class).
Renders logo header, permission-filtered nav sections, and user/logout
footer. Not yet wired — next task mounts it in a Drawer.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>