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/useToast.ts

@@ -13,8 +13,19 @@ const toasts = ref<Toast[]>([])
 const MAX_TOASTS = 3
 let nextId = 1
 
+// Anti-doublon : ignore un toast identique affiché dans les 2 dernières secondes
+const recentMessages = new Map<string, number>()
+const DEDUP_WINDOW = 2000
+
 export function useToast() {
   const showToast = (message: string, type: ToastType = 'info', duration = 3500): number => {
+    const dedupKey = `${type}::${message}`
+    const lastShown = recentMessages.get(dedupKey)
+    if (lastShown && Date.now() - lastShown < DEDUP_WINDOW) {
+      return -1
+    }
+    recentMessages.set(dedupKey, Date.now())
+
     const id = nextId++
     const toast: Toast = {
       id,