Allow customization of width of LLM messages in `LlmMessageView`

[zugzwangm]

The current width of each LLM message in the LlmMessageView component appears too narrow for certain use cases. Please allow users to configure the flex values inside LlmMessageView/Row that determine the message width.

[csells]

That's a good feature request, @zugzwangm. That's out of scope for this version, but feel free to submit a PR.