I understand that the whole backend of a dapp can exist on the blockchain, but it still seems like you need something to serve the frontend UI to make it realistically usable.
Are there any standards or protocols that allow the hosting of frontends to exist on the blockchain, so that one could simply load the entirety of a Dapp via the smart contract address?
Technically speaking a Dapp is decentralised as long as it's logic is in a smart contract onchain. You can serve a front-end to interact with the contract but if I know the contract ABI I could technically interact with it simply through transactions.
Regardless the main effort currently in the direction you are talking about is something called Swarm. Swarm plans to use a torrent-like distribution service to host files required for Dapps, at some point this will hopefully be used to distribute the files required for a front-end.
