message: CR: Handle responses with *no* Content-Type
  Can happen with extant `_handler.js` that does not specify this
  header *and* doesn't work.
diff: @@ -47,7 +47,8 @@ export const AuthProvider: FC = ({ children }) => {
  if (!req.ok) {
  throw new Error(`Failed to get /whoami ${req.statusText}`);
  }
  - if (!req.headers.get('content-type')!.startsWith('application/json')) {
  + const contentType = req.headers.get('content-type');
  + if (!(contentType && contentType.startsWith('application/json'))) {
  throw new Error('Failed to GET /whoami JSON data: backend may be missing file _handler.js.');
  }
  return req.json();
  