fix(m2-m3): data-testid on new-client+add-client btns; fix client subscriptions API response shape; update E2E specs

app/(app)/clients/[id]/page.tsx

@@ -55,7 +55,7 @@ export default function ClientDetailPage() {
 
   const { data: subscriptions, isError: subsError } = useQuery<Subscription[]>({
     queryKey: ["client-subscriptions", id],
-    queryFn: async () => { const r = await api.get<Subscription[]>(`/api/v1/clients/${id}/subscriptions`); return r.data; },
+    queryFn: async () => { const r = await api.get<{ data: Subscription[] }>(`/api/v1/clients/${id}/subscriptions`); return (r.data as any).data ?? []; },
     enabled: activeTab === "subscriptions",
   });