feat(admin): add permission-based filtering to admin navigation

- add optional `permission` field to nav items in registry
- filter nav items by user permissions in `buildNavigationSections`
- auto-hide sections when all their items are filtered out
- fetch user permissions in `AdminLayout.server.js` and pass to navigation builder
- update docs and README to document `permission` param and new signature

src/features/admin/registry.js

@@ -57,8 +57,8 @@ export function registerNavSection({ id, title, icon, order = 0 }) {
   navSections.set(id, { id, title, icon, order });
 }
 
-export function registerNavItem({ id, label, icon, href, order = 0, sectionId = 'main', position }) {
-  navItems.set(id, { id, label, icon, href, order, sectionId, position });
+export function registerNavItem({ id, label, icon, href, order = 0, sectionId = 'main', position, permission }) {
+  navItems.set(id, { id, label, icon, href, order, sectionId, position, permission });
 }
 
 export function getNavSections() {