On screencast, mute notifications or enable privacy mode

### Feature Description

We already have screencast detection logic, would be nice to add a toggle for muting notifications/enabling privacy mode(block out contents from notif popups) automatically.

### Use Case

_No response_

### Compositor(s)

All compositors

### If Other, please specify

_No response_

### Proposed Solution

_No response_

### Alternatives/Existing Solutions

_No response_

Start by locating the existing screencast detection logic and the notification privacy or muting behavior. Determine how the toggle should connect these pieces for all supported compositors, then verify that screencasting automatically mutes notifications or blocks their contents.

Written by the indexing model from the issue text.