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

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

frontend/services/dto/client.ts

@@ -0,0 +1,19 @@
+export type Client = {
+    id: number
+    '@id'?: string
+    name: string
+    email: string | null
+    phone: string | null
+    street: string | null
+    city: string | null
+    postalCode: string | null
+}
+
+export type ClientWrite = {
+    name: string
+    email: string | null
+    phone: string | null
+    street: string | null
+    city: string | null
+    postalCode: string | null
+}