fix: resolve React Error #310 and refactor admin section

- Fix React bug #33580: remove Suspense boundaries co-located with Link components
- Delete settings/loading.tsx and admin/loading.tsx (root cause of race condition)
- Convert all admin navigation from Next.js Link to anchor tags
- Move admin pages to dedicated (admin) route group
- Add AdminHeader matching main header visual design
- Add AdminSidebar with anchor-based navigation
- Add /api/admin/models route handler (replaces server actions for GET)
- Add /api/debug/client-error for server-side browser error reporting
- Add useNoteRefreshOptional() to fix crash in AdminHeader
- Hide Admin Dashboard menu for non-admin users
- Change app icons from yellow to blue (#3A7CA5) matching brand primary
- Fix admin search bar width to match main header

Made-with: Cursor

memento-note/components/header.tsx

@@ -58,6 +58,7 @@ export function Header({
 
   const noSidebarMode = ['/agents', '/chat', '/lab'].some(r => pathname.startsWith(r))
 
+
   // Track last pushed search to avoid infinite loops
   const lastPushedSearch = useRef<string | null>(null)
 
@@ -421,12 +422,16 @@ export function Header({
                   <span>{t('settings.profile') || 'Profile'}</span>
                 </Link>
               </DropdownMenuItem>
-              <DropdownMenuItem asChild className="cursor-pointer">
-                <Link href="/admin">
-                  <Shield className="mr-2 h-4 w-4" />
-                  <span>{t('nav.adminDashboard')}</span>
-                </Link>
-              </DropdownMenuItem>
+              {(currentUser as any)?.role === 'ADMIN' && (
+                <DropdownMenuItem asChild className="cursor-pointer">
+                  {/* Force hard reload: client-side navigation between (main) and (admin)
+                      route groups triggers React #310 in Next.js 16.x (framework bug). */}
+                  <a href="/admin">
+                    <Shield className="mr-2 h-4 w-4" />
+                    <span>{t('nav.adminDashboard')}</span>
+                  </a>
+                </DropdownMenuItem>
+              )}
               <DropdownMenuItem onClick={() => signOut()} className="cursor-pointer text-red-600 focus:text-red-600">
                 <LogOut className="mr-2 h-4 w-4" />
                 <span>{t('auth.signOut') || 'Sign out'}</span>