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/pieces/create.vue

@@ -315,6 +315,7 @@ import SearchSelect from '~/components/common/SearchSelect.vue'
 import { usePieceTypes } from '~/composables/usePieceTypes'
 import { usePieces } from '~/composables/usePieces'
 import { useToast } from '~/composables/useToast'
+import { humanizeError } from '~/shared/utils/errorMessages'
 import { useCustomFields } from '~/composables/useCustomFields'
 import { useDocuments } from '~/composables/useDocuments'
 import { formatPieceStructurePreview } from '~/shared/modelUtils'
@@ -599,7 +600,7 @@ const submitCreation = async () => {
       toast.showError(result.error)
     }
   } catch (error: any) {
-    toast.showError(error?.message || 'Erreur lors de la création de la pièce')
+    toast.showError(humanizeError(error?.message) || 'Impossible de créer la pièce')
   } finally {
     submitting.value = false
     uploadingDocuments.value = false