fix: don't send Content-Type: application/json with empty POST body

apps/web/src/lib/api.ts

@@ -82,10 +82,12 @@ export async function apiGet<T>(path: string): Promise<T> {
 }
 
 export async function apiPost<T>(path: string, body?: unknown): Promise<T> {
+  const headers =
+    body !== undefined ? formatHeaders({ "Content-Type": "application/json" }) : formatHeaders();
   const res = await fetch(`${API_BASE}${path}`, {
     method: "POST",
-    headers: formatHeaders({ "Content-Type": "application/json" }),
-    body: body ? JSON.stringify(body) : undefined,
+    headers,
+    body: body !== undefined ? JSON.stringify(body) : undefined,
   });
   if (!res.ok) await throwWithMessage(res);
   return res.json();