Add an option to opt out of YouTube Watch Together

## Feature request

Add an option for a participant to opt out of **YouTube Watch Together**.

## Motivation

A meeting participant may want to remain in the call without participating in the shared YouTube viewing experience.

## Proposed behavior

Provide a clear control that lets an individual participant disable or leave YouTube Watch Together without leaving the meeting.

## Acceptance criteria

- Participants can opt out of YouTube Watch Together independently.
- Opting out does not disconnect the participant from the meeting.
- The control is easy to find from the Watch Together experience.

Start from the existing YouTube Watch Together experience and trace how participant viewing state and its controls are handled. Add an individual opt-out control that preserves meeting participation, then verify that it is easy to find and satisfies each acceptance criterion.

Written by the indexing model from the issue text.