✨ 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>

frontend/app/pages/contact.vue

@@ -136,6 +136,7 @@ definePageMeta({
 })
 
 const { t } = useI18n()
+const router = useRouter()
 const { setPageMeta } = useSeo()
 const config = useRuntimeConfig()
 const progressionStore = useProgressionStore()
@@ -194,7 +195,8 @@ async function handleSubmit() {
       },
     })
 
-    isSubmitted.value = true
+    // Rediriger vers le quiz bonus
+    router.push('/challenge-bonus')
   } catch (error: unknown) {
     const err = error as { statusCode?: number }
     if (err.statusCode === 429) {