refactor(catalog) : address Matthieu review on composables (constant + shared helpers)

- useCategoriesAdmin : extract `HYDRA_NO_PAGINATION = 999` to a named
  constant (was duplicated between fetchAll and fetchTypes) + comment
  the post-M0 server-pagination debt.

- useCategoryForm + useApi + shared/utils/api : drop the local copy of
  `extractErrorMessage` in useCategoryForm (it was duplicating the one
  in useApi), and centralize Hydra error / violation extraction in
  `shared/utils/api.ts` via two new helpers :
    - extractApiErrorMessage(data) : tries hydra:description, detail,
      description, message, error, title, hydra:title — used by both
      useApi.onResponseError and useCategoryForm.handleApiError.
    - extractApiViolations(data) : returns the ApiPlatform 422
      violations as a typed array (handles `violations` and
      `hydra:violations`), letting each caller map them onto its own
      fields. useCategoryForm now uses this helper instead of an
      inline loop, ready for the next form drawer to reuse.

handleApiError keeps a manual fallback toast on non-409/422 errors :
the native useApi toast is disabled by design (`toast: false`) to allow
fine-grained 409/422 mapping, so the catch-all branch must re-emit one
or a 500 would be silent.

No behavior change — 43/43 Vitest tests still pass.

frontend/shared/utils/api.ts

@@ -31,3 +31,62 @@ export interface HydraCollection<T> {
 export function extractHydraMembers<T>(collection: HydraCollection<T>): T[] {
     return collection.member ?? []
 }
+
+/**
+ * Une violation de contrainte API Platform (reponse 422). Le `propertyPath`
+ * pointe le champ concerne, `message` est le libelle a afficher.
+ */
+export interface ApiViolation {
+    propertyPath: string
+    message: string
+}
+
+/**
+ * Extrait les violations d'un payload d'erreur 422 d'API Platform 4. Supporte
+ * les deux formats de negociation (`violations` ou `hydra:violations`) et
+ * renvoie un tableau vide si le payload n'en contient pas d'exploitables.
+ *
+ * Utilise par useCategoryForm et tout futur composable de formulaire qui
+ * doit mapper les violations serveur sur ses champs.
+ */
+export function extractApiViolations(data: unknown): ApiViolation[] {
+    if (!data || typeof data !== 'object') return []
+    const record = data as Record<string, unknown>
+    const raw = record.violations ?? record['hydra:violations']
+    if (!Array.isArray(raw)) return []
+    const out: ApiViolation[] = []
+    for (const v of raw) {
+        if (!v || typeof v !== 'object') continue
+        const obj = v as Record<string, unknown>
+        out.push({
+            propertyPath: String(obj.propertyPath ?? ''),
+            message: String(obj.message ?? ''),
+        })
+    }
+    return out
+}
+
+/**
+ * Extrait un message d'erreur lisible depuis un payload Hydra / JSON
+ * d'erreur API Platform. Essaie les champs courants dans l'ordre :
+ * `hydra:description` → `detail` → `description` → `message` → `error` →
+ * `title` → `hydra:title`. Renvoie '' si rien d'exploitable.
+ *
+ * Si `data` est une string, la renvoie telle quelle (cas des erreurs
+ * Symfony en text/plain ou des messages bruts).
+ */
+export function extractApiErrorMessage(data: unknown): string {
+    if (typeof data === 'string') return data
+    if (!data || typeof data !== 'object') return ''
+    const record = data as Record<string, unknown>
+    return (
+        (record['hydra:description'] as string)
+        ?? (record.detail as string)
+        ?? (record.description as string)
+        ?? (record.message as string)
+        ?? (record.error as string)
+        ?? (record.title as string)
+        ?? (record['hydra:title'] as string)
+        ?? ''
+    )
+}