test: add data-testid to mode buttons, admin button, and quantity controls

frontend/components/BottomNav.tsx

@@ -60,6 +60,7 @@ export default function BottomNav({
         {/* Admin Settings */}
         {currentUser?.role === 'admin' && (
           <button
+            data-testid="admin-button"
             onClick={() => router.push('/admin')}
             aria-label="Go to Admin"
             className={cn("flex flex-col items-center gap-1 rounded px-2 py-1 cursor-pointer transition-colors focus:ring-2 focus:ring-primary focus:outline-none", isAdmin && "text-primary")}