Auto-add `/` or throw error for `proxyBaseEndpoint` without start `/`

[maximepvrt]

If proxyBaseEndpoint is configured without a leading /, some requests may fail silently. For example, in my case, only the homepage was tracked correctly, while other pages were not.

Proposal:

Automatically prepend a / to proxyBaseEndpoint if missing, or throw an error/warning if the value does not start with /.