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/useApi.ts

@@ -1,4 +1,5 @@
 import { useToast } from './useToast'
+import { humanizeError, extractApiErrorMessage } from '~/shared/utils/errorMessages'
 
 export interface ApiResponse<T = any> {
   success: boolean
@@ -59,23 +60,26 @@ export function useApi() {
       } else {
         const contentType = response.headers.get('content-type') || ''
         let errorData: Record<string, unknown> = {}
-        if (contentType.includes('application/json')) {
+        if (contentType.includes('json')) {
           errorData = await response.json().catch(() => ({}))
         } else {
           const text = await response.text().catch(() => '')
           errorData = text ? { message: text } : {}
         }
-        const errorMessage = response.status === 403
+        const rawMessage = response.status === 403
           ? 'Permissions insuffisantes pour cette action.'
-          : (errorData.message as string) || `Erreur ${response.status}: ${response.statusText}`
+          : extractApiErrorMessage(errorData) || `Erreur ${response.status}: ${response.statusText}`
+        const errorMessage = humanizeError(rawMessage)
         showError(errorMessage)
         return { success: false, error: errorMessage, status: response.status }
       }
     } catch (error) {
       clearTimeout(timeoutId)
       const err = error as Error & { name?: string }
-      const errorMessage = err.name === 'AbortError' ? 'Timeout de la requête' : err.message || 'Erreur réseau'
-      showError(`Erreur réseau: ${errorMessage}`)
+      const errorMessage = err.name === 'AbortError'
+        ? 'La requête a pris trop de temps. Veuillez réessayer.'
+        : 'Impossible de contacter le serveur. Vérifiez votre connexion.'
+      showError(errorMessage)
       return { success: false, error: errorMessage }
     }
   }