[Story] Hide Notification Balloon

### Story

As an everyday user, I want to be able to hide notification balloons, without muting the entire channel, so my OCD can get some rest.

By balloon, I'm referencing the white dot on the right side:

I still want to be notified about mentions and keywords:

But I do not want to see the balloon. Eg. Same when there are no new messages:

This is a soft mute of some sort, same as Slack when the channel turns dark gray, but shows a balloon on tagged notifications:

Or Discord when the muted channel shows no balloons regarding new messages, but shows a balloon on tagged notifications:

### Dependencies

- TBD

### Sign-offs

#### Android
- [ ] Design sign-off on completion
- [ ] QA sign-off on completion
- [ ] Product sign-off on completion

#### iOS
- [ ] Design sign-off on completion
- [ ] QA sign-off on completion
- [ ] Product sign-off on completion

#### Web
- [ ] Design sign-off on completion
- [ ] QA sign-off on completion
- [ ] Product sign-off on completion

### Scope

```[tasklist]
### Android
- [ ] Verified with build in screen reader
```

```[tasklist]
### iOS
- [ ] Verified with build in screen reader
```

```[tasklist]
### Web
- [ ] Verified with system in screen reader
```

## Out of scope

- TBD

The issue names no files, tests, or entry points; begin by locating notification-balloon behavior separately in the Android, iOS, and Web clients. Use the screenshots and stated soft-mute behavior to clarify the platform-specific settings and acceptance criteria. Done means ordinary message balloons can be hidden while mentions and keywords still produce notifications, with screen-reader verification on each platform.

Written by the indexing model from the issue text.