fix: MCQ clicks broken — restore onclick in showMCQ, remove duplicate addEventListener

btn.onclick in showMCQ is reliable (set fresh per question).
The addEventListener in wireButtons had timing issues with the _pp guard.
Keep mcqAnswered guard to prevent any double-count.

app.py

@@ -573,7 +573,7 @@ BRIDGE_JS = """() => {
       if (txt) txt.textContent = (data.choices && data.choices[l]) || '';
       if (btn) {
         btn.className = 'mcq-btn'; btn.disabled = false;
-        btn.onclick = null;
+        btn.onclick = ((letter) => () => handleMCQAnswer(letter))(l);
       }
     }
     $('mcq-options')?.classList.remove('hidden');
@@ -711,10 +711,6 @@ BRIDGE_JS = """() => {
         if(qc) qc.scrollIntoView({behavior:'smooth',block:'start'});
       }],
     ];
-    for (const l of ['A','B','C','D']) {
-      const btn = $('mcq-'+l.toLowerCase());
-      if (btn && !btn._pp) { btn._pp=true; btn.addEventListener('click', () => handleMCQAnswer(l)); }
-    }
     for (const [id, fn] of pairs) {
       const el = $(id);
       if (el && !el._pp) { el._pp = true; el.addEventListener('click', fn); }