http: tweak http timeouts

change --http-timeout default to 15000 (up from 5000)
change --http-connect-timeout default to 8000 (down from 300000, curl's default)

5 second _total_ transfer time can be a little tight. I generally don't see a
good reason to overly limit this value.

Worth noting that help.zon said the default for --http-timeout was 10000, but
it was, in fact, 5000.

Hopefully this improve situations like https://github.com/lightpanda-io/browser/issues/3395
which I believe are due to slow proxies.