AI Disclaimer: move timestamp logic to the clientside

At the moment the AI disclaimer splash screen will show once every 24 hours, using a server-side timestamp

This doesn't strictly meet the design, which says that the disclaimer splash should be shown once per day.

The problem here is timezones 🎉 The server doesn't know the user's timezone so doesn't know what day it is for the user.

My preferred solution here would be to move the timestamping logic into the client, writing it to local storage. This WILL reflect the user's actual day (assuming they're not on a plane). The caveat here is that if you switch device or browser, you'll get multiple popups a day 🤷

We might want to wait for the react rewrite of the frontend before doing this work

Start with the current AI disclaimer splash implementation and review how its server-side timestamp is recorded. Compare this with the planned React frontend and determine where client-side local storage should track the user's calendar day. Done means the splash appears once per local day, with the device or browser limitation documented or accepted.