feat: answer field UX improvements

- Auto-add one empty answer field when opening a blank question form
- Auto-append new empty field when typing in the last answer field
- Strip empty answer rows before submit (novalidate + JS cleanup)
- Tab key skips correct/delete buttons, jumping straight to next answer

templates/backoffice/partials/answer_row.html.twig

@@ -4,13 +4,13 @@
         <span class="text-muted" data-drag-handle style="cursor: grab" title="{{ 'Drag to reorder'|trans }}"><i class="bi bi-grip-vertical"></i></span>
         <div class="flex-grow-1">{{ form_widget(answerForm.text) }}</div>
         <div class="d-none">{{ form_widget(answerForm.isRightAnswer) }}</div>
-        <button type="button"
+        <button type="button" tabindex="-1"
                 class="btn btn-sm {{ answerForm.isRightAnswer.vars.checked ? 'btn-success' : 'btn-danger' }}"
                 title="{{ 'Toggle correct answer'|trans }}"
                 onclick="var cb=this.closest('[data-collection-item]').querySelector('input[type=checkbox]');cb.checked=!cb.checked;this.classList.toggle('btn-success',cb.checked);this.classList.toggle('btn-danger',!cb.checked);this.querySelector('i').className=cb.checked?'bi bi-check-lg':'bi bi-x-lg'">
             <i class="{{ answerForm.isRightAnswer.vars.checked ? 'bi bi-check-lg' : 'bi bi-x-lg' }}"></i>
         </button>
-        <button type="button" class="btn btn-sm btn-outline-danger"
+        <button type="button" tabindex="-1" class="btn btn-sm btn-outline-danger"
                 data-action="bo--form-collection#removeItem"><i class="bi bi-trash"></i></button>
     </div>
 {% endmacro %}