Add an audio-only mode for Conclave meetings

## Overview

Add an audio-only mode so participants with slow or limited internet connections can join and participate without sending or receiving video.

## Issue type

Feature request

## Details

Provide a meeting-level or participant-selectable audio-only experience that disables video while preserving core meeting participation, including audio and chat. The mode should be easy to enable before or during a meeting and clearly indicate that video is off.

## Acceptance criteria

- [ ] Participants can choose an audio-only mode before joining or while in a meeting.
- [ ] Audio-only mode disables the participant's outgoing video and avoids loading/receiving remote video where applicable.
- [ ] Participants can still use meeting audio and chat.
- [ ] The UI clearly shows that audio-only mode is active.

## Additional context

Requested in the meeting due to a participant reporting a slow internet connection and being unable to enable their camera.

_Created by the Conclave in-meeting assistant._

Start by tracing the web/mobile meeting join and in-meeting media controls, then follow how video is sent, received, and rendered while audio and chat remain available. Confirm the existing entry points before deciding whether the mode belongs at meeting or participant level; done means all listed acceptance criteria work before and during a meeting, with clear audio-only status.

Written by the indexing model from the issue text.