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/machines/index.vue

@@ -138,6 +138,7 @@ import { useMachines } from '~/composables/useMachines'
 import { useSites } from '~/composables/useSites'
 import { useMachineTypesApi } from '~/composables/useMachineTypesApi'
 import { useToast } from '~/composables/useToast'
+import { humanizeError } from '~/shared/utils/errorMessages'
 import IconLucidePlus from '~icons/lucide/plus'
 import IconLucideFactory from '~icons/lucide/factory'
 import IconLucideMapPin from '~icons/lucide/map-pin'
@@ -214,10 +215,10 @@ const confirmDeleteMachine = async (machine) => {
       if (result.success) {
         showSuccess(`Machine "${machine.name}" supprimée avec succès`)
       } else {
-        showError(`Erreur lors de la suppression: ${result.error}`)
+        showError(`Impossible de supprimer la machine : ${result.error}`)
       }
     } catch (error) {
-      showError(`Erreur lors de la suppression: ${error.message}`)
+      showError(`Impossible de supprimer la machine : ${humanizeError(error.message)}`)
     }
   }
 }