Idea: move `auth.config.ts` to respective `app/` and `server/` directories

Wondering what you think about this:

```bash
# before
auth.config.ts
auth-client.config.ts
package.json
```

```bash
# after
app/
  auth.config.ts
server/
  auth.config.ts
package.json
```

I did not yet use your module so it might create issues or not, but happy to hear your thoughts on it.

BTW, the docs is awesome!