But doesn't the frontend also fit for being a community asset?

Not really because it can't be held by the blockchain accounts. I am just willing to give it to someone else if the community thought they'd be a better custodian of it. There can be multiple frontends though. You could spin up simplysports.com and use SPORTS your own way. We used to run sportstalksocial.com for the outpost and sportstalk.social for the old style.

is it only the things that are onchain that belong to the community?

More or less. Unless there's a legal entity how would a blockchain own other assets?

reason would be the payment for that work

Payments should be handled by token fund requests. If a token fund request receives support beyond the funding limit then it will payout on day one. Additionally payment proposals can be made on a future date to build support.