Openrouter has a chat built in as well.
Also, since librechat was mentioned, the self hosted option I currently prefer is openwebui - connected to openrouter and gemini here.
Not sure if you noticed, but the first CTA on the link you posted is a prompt to "Start a message", which then opens a multi-model capable chat interface.
https://openrouter.ai/
edited to remove statement saying API only, as per comments.