fix(errors) : humanize backend error messages for end users

Add centralized error translation layer (humanizeError) that converts
raw Symfony/Doctrine/API Platform messages into user-friendly French.
Fix useApi to extract errors from all backend response formats
(violations, error, message, hydra:description, detail).
Add toast deduplication to prevent double display. Replace error toast
icon (X → CircleX) to distinguish from the dismiss button.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/pages/product/[id]/edit.vue

@@ -407,6 +407,7 @@ import DocumentPreviewModal from '~/components/DocumentPreviewModal.vue'
 import { useProducts } from '~/composables/useProducts'
 import { useCustomFields } from '~/composables/useCustomFields'
 import { useToast } from '~/composables/useToast'
+import { humanizeError } from '~/shared/utils/errorMessages'
 import { useDocuments } from '~/composables/useDocuments'
 import { useConstructeurs } from '~/composables/useConstructeurs'
 import { useProductHistory, type ProductHistoryEntry } from '~/composables/useProductHistory'
@@ -700,7 +701,7 @@ const submitEdition = async () => {
       await router.push('/product-catalog')
     }
   } catch (error: any) {
-    toast.showError(error?.message || 'Erreur lors de la mise à jour du produit')
+    toast.showError(humanizeError(error?.message) || 'Impossible de mettre à jour le produit')
   } finally {
     saving.value = false
   }