✨ feat(frontend): quiz bonus post-contact (Story 4.9)

- Add BonusQuiz.vue component with 7 randomized questions
- Add challenge-bonus.vue page with intro, quiz, and results
- Redirect to bonus quiz after successful contact form submission
- Add i18n translations for bonus.* (fr/en)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/implementation-artifacts/4-9-challenge-post-formulaire.md

@@ -1,6 +1,6 @@
 # Story 4.9: Challenge post-formulaire
 
-Status: ready-for-dev
+Status: done
 
 ## Story