feat: add ArgoCD Application and k8s manifests for frontend

- application.yaml: ArgoCD Application resource (manual sync for now,
  same pattern as nextgen-be)
- manifests/: namespace, deployment, service, ingress

manifests/00-namespace.yaml

@@ -0,0 +1,4 @@
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: nextgen-fe