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

src/Repository/QuizCandidateRepository.php

@@ -68,4 +68,15 @@ class QuizCandidateRepository extends ServiceEntityRepository
         $quizCandidate->penaltySeconds = $penalty;
         $this->getEntityManager()->flush();
     }
+
+    public function resetProgressForCandidate(Quiz $quiz, Candidate $candidate): void
+    {
+        $quizCandidate = $this->findOneBy(['candidate' => $candidate, 'quiz' => $quiz]);
+        if (!$quizCandidate instanceof QuizCandidate) {
+            return;
+        }
+
+        $quizCandidate->started = null;
+        $this->getEntityManager()->flush();
+    }
 }