Ability to restart an historical chat

Currently, the application does maintain chat history but they all become inactive (can be prompted) once a new chat is started.It would be a great feature to allow users to continue previous conversations as well.

Start by tracing how the Chainlit application stores chat history and how conversation state is sent to Amazon Bedrock. Confirm how a historical chat is selected and resumed after starting a new chat. Done means users can prompt a previously stored conversation without losing or mixing its context.

Written by the indexing model from the issue text.