Add reset progress action for a candidate's quiz attempt, and modal-based candidate editing (#204)

* feat: add reset progress action for a candidate's quiz attempt

Admins previously had no way to let a candidate retake a quiz once
started. Adds a backoffice action that clears a candidate's given
answers and start timestamp for a quiz, closes #20.

* fix: wrap candidate progress reset in a transaction

Avoids leaving a candidate half-reset (answers deleted but started
still set) if the second flush fails. Addresses CodeRabbit review
comment on PR #204.

* feat: open candidate edit/add forms in modals with dirty-aware dismissal

Rename-candidate modal always blocked backdrop-click dismissal via a
hardcoded static backdrop; wire it into the existing bo--modal
controller so outside clicks/Escape only get blocked once the name
field has actually been edited. Also move "Add Candidate" from a
full-page navigation into the same turbo-frame modal pattern already
used for editing quiz questions, keeping the full-page form as a
fallback for non-JS/turbo requests.

* feat: show a hint for the candidates textarea and gate translations in CI

Add a help hint on the add-candidates form explaining one candidate per
line, and translate it plus a few strings from the earlier reset-progress
feature that had been missed. Also add a CI step that re-runs
translation:extract and fails the build if it produces uncommitted
changes, so untranslated/stale strings can't slip through review again.

* Just links

templates/backoffice/quiz/tab_candidates_list.html.twig

@@ -37,7 +37,7 @@
                     {% endif %}
                 </td>
                 <td>
-                    <form action="{{ path('tvdt_backoffice_toggle_candidate', {quiz: quiz.id, candidate: candidate.id}) }}" method="POST">
+                    <form action="{{ path('tvdt_backoffice_toggle_candidate', {quiz: quiz.id, candidate: candidate.id}) }}" method="POST" class="d-inline">
                         <input type="hidden" name="_token" value="{{ csrf_token('toggle_candidate') }}">
                         <button type="submit" class="btn btn-sm btn-outline-secondary">
                             {% if quizCandidate == null or quizCandidate.active %}
@@ -47,6 +47,12 @@
                             {% endif %}
                         </button>
                     </form>
+                    {% if quizCandidate and quizCandidate.started %}
+                        <form action="{{ path('tvdt_backoffice_reset_candidate_progress', {quiz: quiz.id, candidate: candidate.id}) }}" method="POST" class="d-inline" onsubmit="return confirm('{{ 'Are you sure you want to reset progress for this candidate? Their given answers for this quiz will be deleted.'|trans|e('js') }}');">
+                            <input type="hidden" name="_token" value="{{ csrf_token('reset_candidate_progress') }}">
+                            <button type="submit" class="btn btn-sm btn-outline-danger">{{ 'Reset progress'|trans }}</button>
+                        </form>
+                    {% endif %}
                 </td>
             </tr>
         {% endfor %}