Add support for local avatars (using core `get_avatar`) rather than hard-coding Gravatars

Currently only Gravatars are supported. If a Gravatar for a user doesn't exist, the plugin reverts to the default image, rather than searching for a local avatar.

Start by tracing where the plugin chooses a Gravatar or default image, then read WordPress core's get_avatar behavior. Update the avatar lookup so a local avatar is considered when no Gravatar exists, and verify that users without Gravatars display the local avatar instead of the default image.

Written by the indexing model from the issue text.