[Feature Request] Show the user's profile picture from the OIDC `picture` claim

Description When a user signs in through OpenID Connect, show their profile picture in the user menu, in place of the fixed person-circle icon. The picture should come from the standard picture claim that the identity provider already returns. This was requested before in #11432 (5 upvotes). The bot closed that thread for inactivity and then locked it, so this is a fresh thread for the same request. If you upvoted #11432, please upvote here. Why it is a small change. The data is already stored. Paperless signs users in through django-allauth, and allauth's OpenID Connect provider keeps the full userinfo response, picture included, in SocialAccount.extra_data. ProfileSerializer already serializes the user's social accounts (SocialAccountSerializer in src/paperless/serialisers.py), so a sketch of the change is: 1. Backend: add a read-only avatar_url to the profile, or to SocialAccountSerializer, taken from extra_data (the userinfo picture, falling back to the ID token'…