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/useMachineCreatePage.ts

@@ -15,6 +15,7 @@ import { usePieces } from '~/composables/usePieces'
 import { useProducts } from '~/composables/useProducts'
 import { useApi } from '~/composables/useApi'
 import { useToast } from '~/composables/useToast'
+import { humanizeError } from '~/shared/utils/errorMessages'
 import { useMachineCreateSelections } from '~/composables/useMachineCreateSelections'
 import {
   useMachineCreatePreview,
@@ -365,10 +366,10 @@ export function useMachineCreatePage() {
         clearRequirementSelections()
         await navigateTo('/machines')
       } else if (result.error) {
-        toast.showError(`Impossible de créer la machine: ${result.error}`)
+        toast.showError(`Impossible de créer la machine : ${humanizeError(result.error)}`)
       }
     } catch (error: any) {
-      toast.showError(`Erreur lors de la création: ${error.message}`)
+      toast.showError(`Impossible de créer la machine : ${humanizeError(error.message)}`)
     } finally {
       submitting.value = false
     }