Avatar image is broken when reading threads from history

[LucasMagnum]

Describe the bug

The PR https://github.com/Chainlit/chainlit/pull/1326 introduces a bug when it expects the get_avatar to match string (https://github.com/Chainlit/chainlit/pull/1326/files#diff-9a73e67e163edc3fae6839bf2bcc8c6ab650624eff6668502388a7a86b7ece78R964) .

The Frontend app can make requests to the avatar that do not match the Regex (https://github.com/Chainlit/chainlit/blob/main/frontend/src/components/molecules/messages/components/Avatar.tsx#L31) this creates a bug for existing applications where the config.ui.name contains space (e.g. My Assistant).

To Reproduce Set your config.ui.name to My Assistant

1. Create a thread
2. Publish messages
3. Create a new thread
4. Try to resume the previous thread
5. See the 400 error and the avatar not showing

Expected behavior Avatar should be loaded for threads in history.