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/components/ToastContainer.vue

@@ -24,7 +24,7 @@
                 class="w-4 h-4"
                 aria-hidden="true"
               />
-              <IconLucideX
+              <IconLucideCircleX
                 v-else-if="toast.type === 'error'"
                 class="w-4 h-4"
                 aria-hidden="true"
@@ -64,6 +64,7 @@
 import { useToast } from '~/composables/useToast'
 import IconLucideCheck from '~icons/lucide/check'
 import IconLucideX from '~icons/lucide/x'
+import IconLucideCircleX from '~icons/lucide/circle-x'
 import IconLucideAlertTriangle from '~icons/lucide/alert-triangle'
 import IconLucideInfo from '~icons/lucide/info'