Add React race hub frontend

frontend/src/api.ts

@@ -0,0 +1,9 @@
+import type { RaceHub } from './types'
+
+export async function fetchRaceHub(sessionKey: number): Promise<RaceHub> {
+  const res = await fetch(`/api/v1/race-hub?session_key=${sessionKey}`)
+  if (!res.ok) {
+    throw new Error(`API ${res.status}: ${res.statusText}`)
+  }
+  return res.json()
+}