Summary: Add an option to disable the chat announcement message when AI Notetaker joins a meeting.

Current behavior: When AI Notetaker joins a call, it always posts an introduction message in the meeting chat. The message text can be customized but cannot be turned off entirely (it defaults back if left empty).

Requested behavior: A toggle in AI Notetaker settings (under Personalization) to enable "Silent Join," where the bot joins and records without posting any message in the meeting chat.

Use case: In some meetings, particularly with external clients or senior stakeholders, the chat announcement can feel disruptive or draw unnecessary attention. The Zoom recording disclaimer and visual indicator already inform participants that recording is active, making the additional chat message redundant in those scenarios.