marizeMode() {
        const diff = els.difficulty.value;
        const ops = getSelectedOps().map(k => symbols[k]).join(' ');
        const dur = parseInt(els.duration.value, 10);
        els.modeSummary.textContent = `Mode: ${diff} • Ops: ${ops} • ${dur}s`;
      }

      function randInt(min, max) {
        return Math.floor(Math.random() * (max - min + 1)) + min;
      }

      function generateProblem() {
        const diff = els.difficulty.value;
        const ops = getSelectedOps();
        const