Avatars from OpenID Connect

### Describe the problem

OpenID Connect has a possibility to include a link to the user avatar.

Weblate should use that function to unify avatars across services.

### Solution brainstorm

Weblate should check avatar claim and set the avatar appropriately.

### Describe alternatives you have considered

There is no alternatives.

### Screenshots

_No response_

### Additional context

_No response_

Start by locating Weblate’s OpenID Connect authentication flow and the existing user-avatar handling. Trace how avatar claims are read and persisted, then verify that a configured OpenID Connect avatar is used consistently across services; the issue does not name specific files or tests.

Written by the indexing model from the issue text.