feat: harden runtime readiness and session workflow

frontend/src/api/client.ts

@@ -12,7 +12,11 @@ export async function apiFetch<T>(path: string, init?: RequestInit): Promise<T>
   }
   const res = await fetch(joinBackendPath(BASE, path), { ...init, headers });
   if (!res.ok) throw new Error(`${res.status} ${await res.text().catch(() => "")}`);
-  return res.status === 204 ? (undefined as T) : ((await res.json()) as T);
+  if (res.status === 204) return undefined as T;
+  // Accepted fire-and-forget endpoints may legitimately return 202 with no
+  // representation. Keep apiFetch useful for both 202 and 204 contracts.
+  const body = await res.text();
+  return body ? (JSON.parse(body) as T) : (undefined as T);
 }
 
 export { BASE };

frontend/src/api/sessions.test.ts

@@ -1,6 +1,6 @@
 import { http, HttpResponse } from "msw";
 import { server } from "../test/msw";
-import { createSession, listSessions } from "./sessions";
+import { createSession, listSessions, prewarmRuntime } from "./sessions";
 import {
   renameSession, setSessionGroup, archiveSession, unarchiveSession,
   deleteSession, getSessionDocuments,
@@ -18,6 +18,18 @@ test("createSession POSTs only {question} and returns the id", async () => {
   expect(body).toEqual({ question: "q" });
 });
 
+test.each([202, 204])("prewarmRuntime accepts a body-less %s response", async (status) => {
+  let called = false;
+  server.use(
+    http.post("http://localhost:8787/runtime/prewarm", () => {
+      called = true;
+      return new HttpResponse(null, { status });
+    }),
+  );
+  await expect(prewarmRuntime()).resolves.toBeUndefined();
+  expect(called).toBe(true);
+});
+
 test("listSessions GETs the array", async () => {
   server.use(http.get("http://localhost:8787/sessions", () => HttpResponse.json([{ id: "s1", status: "open", question: "q", summary: null, created_at: "t", updated_at: null, author: null }])));
   const rows = await listSessions();

frontend/src/api/sessions.ts

@@ -4,6 +4,10 @@ import type { SessionSummary, SessionDocument, UiResponse } from "./types";
 export const createSession = (i: { question: string; name?: string }) =>
   apiFetch<{ id: string }>("/sessions", { method: "POST", body: JSON.stringify(i) });
 
+/** Best-effort warm-up; callers must not await it before showing the composer. */
+export const prewarmRuntime = () =>
+  apiFetch<void>("/runtime/prewarm", { method: "POST" });
+
 export const listSessions = () => apiFetch<SessionSummary[]>("/sessions");
 
 // eslint-disable-next-line @typescript-eslint/no-explicit-any