feat : add Client DTO, service and Hydra utils (frontend)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/utils/api.ts

@@ -0,0 +1,8 @@
+export type HydraCollection<T> = {
+    'hydra:member': T[]
+    'hydra:totalItems': number
+}
+
+export function extractHydraMembers<T>(response: HydraCollection<T>): T[] {
+    return response['hydra:member'] ?? []
+}