Add unit tests for Avatar component

[SherlockShemol]

The Avatar component in web/app/components/base/avatar doesn't have any test coverage yet.

Since it's a widely used base component, having tests would help us refactor with more confidence.

Planning to cover:

• Basic rendering (image vs fallback initial)
• Props handling (size, className, textClassName)
• Image error fallback behavior
• Edge cases (empty name, CJK characters, unusual sizes)