fix(frontend): only send JSON content-type when a body is present

Body-less POSTs (resume, close) sent content-type: application/json with no
body, so Fastify rejected them with FST_ERR_CTP_EMPTY_JSON_BODY (400) and
clicking an existing session never reopened it. apiFetch now adds the header
only when init.body is set.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/api/client.ts

@@ -1,10 +1,16 @@
 const BASE = import.meta.env.VITE_BACKEND_URL ?? "http://localhost:8787";
 
 export async function apiFetch<T>(path: string, init?: RequestInit): Promise<T> {
-  const res = await fetch(`${BASE}${path}`, {
-    headers: { "content-type": "application/json" },
-    ...init,
-  });
+  // Only declare a JSON content-type when we actually send a body. Body-less
+  // POSTs (resume, close) would otherwise make Fastify reject the empty body
+  // with FST_ERR_CTP_EMPTY_JSON_BODY (400).
+  const headers: Record<string, string> = {
+    ...(init?.headers as Record<string, string> | undefined),
+  };
+  if (init?.body != null && !("content-type" in headers) && !("Content-Type" in headers)) {
+    headers["content-type"] = "application/json";
+  }
+  const res = await fetch(`${BASE}${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);
 }