Send the play queue ID to the server

The server keeps a play queue per client, identified by the X-Queue-Id
header. Send the stored ID with every request and save the ID from every
response, so the client switches to the new queue after a reset or a sync.
The ID is kept in localStorage so a page reload keeps the same queue.

Also send Cache-Control: no-cache, so browsers that cached a random song
before the server sent no-store fetch a new one.

README.md

@@ -25,7 +25,7 @@ The Docker image writes `config.js` from `public/config.template.js` when the co
 
 | Variable | Description |
 |---|---|
-| `API_HOSTNAME` | URL of the MusicServer, for example `https://music.example.com`. Empty to use the same host as the frontend. |
+| `API_HOSTNAME` | URL of the MusicServer, for example `https://music.example.com`. Empty to use the same host as the frontend. On another host, add the frontend's origin to the server's `CORS_ALLOWED_ORIGINS`. |
 | `API_PASSWORD` | The MusicServer's `API_PASSWORD`, used to get an API token. Empty if the server has no password. Anyone who can open the frontend can read it. |
 
 For `npm run serve`, create `public/config.js` (ignored by git):