feat(frontend): per-user DLOB source header

- Add cookie-based source selector (mevnode|drift)\n- Proxy sets x-hasura-dlob-source for HTTP + WS\n- Include same proxy script mount in prod overlay

kustomize/overlays/prod/frontend-graphql-proxy-patch.yaml

@@ -0,0 +1,18 @@
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: trade-frontend
+spec:
+  template:
+    spec:
+      containers:
+        - name: frontend
+          volumeMounts:
+            - name: frontend-server-script
+              mountPath: /app/services/frontend/server.mjs
+              subPath: frontend-server.mjs
+              readOnly: true
+      volumes:
+        - name: frontend-server-script
+          configMap:
+            name: trade-frontend-server-script