Resellers' sidebar was a hardcoded PHP array in
layouts/reseller/header.php with no extension point, unlike admin's
NavbarRegistry/NavbarItem/CoreNavbarProvider. Add a sibling
ResellerNavbarRegistry + ResellerNavbarProviderInterface (mirroring the
existing PermissionRegistry precedent for reseller-specific module
extensions), migrate the hardcoded menu into CoreResellerNavbarProvider,
and wire both into ModuleLoader::bootAll() next to the other
registries. Modules now add a reseller sidebar entry the same way they
already do for admin.
Design: docs/superpowers/specs/2026-09-24-reseller-navbar-registry-design.md
