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/composables/useSiteManagement.ts

@@ -2,6 +2,7 @@ import { computed, onMounted, reactive, ref, watch } from 'vue'
 import { navigateTo, useRoute } from '#imports'
 import { useSites } from '~/composables/useSites'
 import { useToast } from '~/composables/useToast'
+import { humanizeError } from '~/shared/utils/errorMessages'
 import { useDocuments } from '~/composables/useDocuments'
 import { useConfirm } from '~/composables/useConfirm'
 import { getFileIcon } from '~/utils/fileIcons'
@@ -274,10 +275,10 @@ export function useSiteManagement() {
       if (result.success) {
         showSuccess(`Site "${site.name}" supprimé avec succès`)
       } else {
-        showError(`Erreur lors de la suppression: ${result.error}`)
+        showError(`Impossible de supprimer le site : ${humanizeError(result.error)}`)
       }
     } catch (error: any) {
-      showError(`Erreur lors de la suppression: ${error.message}`)
+      showError(`Impossible de supprimer le site : ${humanizeError(error.message)}`)
     }
   }