fix(sidebar): show admin-only nav items to admins regardless of view-as

Filter compared item.roles against effectiveRole, but effectiveRole for
an admin is always 'student' or 'staff' (per AuthContext.tsx:132-137),
so admin-only items (User Management, Settings) were never shown.
Admins now always see admin items based on dbUser.role.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: WangYuTengg

src/client/components/Sidebar.tsx

@@ -111,6 +111,8 @@ export function Sidebar() {
 
   const filteredNavigation = navigation.filter((item) => {
     const role = effectiveRole ?? dbUser.role;
+    // Admins always see admin-only items, regardless of view-as.
+    if (dbUser.role === 'admin' && item.roles.includes('admin')) return true;
     return item.roles.includes(role);
   });