Comply with California Age Verification law

### Problem

New age verification regulation has passed in California and will go into effect January 1st 2027.

https://leginfo.legislature.ca.gov/faces/billTextClient.xhtml?bill_id=202520260AB1043

### Proposal

A high level summary of the requirements is that:

- [ ] during account setup we will be required to indicate the user’s age and
- [ ] use that information to somehow gate content. It sounds like app developers need to be able to access this info.

So my guess is that we’ll need to add a property to the [account portal](https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.portal.Account.html) to satisfy the requirement

### Prior Art (Optional)

Information I’ve seen so far indicates Apple and Google are sorting users into 4 age brackets, so exact age doesn’t seem to be a requirement.

Apple has details of its plan here but I don’t think all of this exact implementation is required by the law: https://developer.apple.com/support/downloads/Helping-Protect-Kids-Online-2025.pdf