fix(docker): harden frontend runtime config

frontend/src/stream/useSessionStream.test.tsx

@@ -13,7 +13,7 @@ beforeEach(() => {
 test("opens an EventSource and feeds NAMED events to the store", () => {
   renderHook(() => useSessionStream("s1"));
   const es = FakeEventSource.instances[0];
-  expect(es.url).toContain("/sessions/s1/events");
+  expect(es.url).toBe("http://localhost:8787/sessions/s1/events");
   // Backend sends `event: ui_request` (named) — drive the addEventListener path
   // that production relies on, not the unnamed onmessage fallback.
   act(() =>

frontend/src/stream/useSessionStream.ts

@@ -1,5 +1,6 @@
 import { useEffect, useState } from "react";
 import { BASE } from "../api/client";
+import { joinBackendPath } from "../api/runtime-config";
 import { useSessionStore } from "../store/sessionStore";
 import type { StreamEvent } from "../api/types";
 
@@ -10,7 +11,7 @@ export function useSessionStream(sessionId: string | null) {
   useEffect(() => {
     if (!sessionId) return;
 
-    const es = new EventSource(`${BASE}/sessions/${sessionId}/events`);
+    const es = new EventSource(joinBackendPath(BASE, `/sessions/${sessionId}/events`));
     es.onopen = () => setConnected(true);
     es.onerror = () => setConnected(false);