Audible notification sound for hidden events in timeline

### Steps to reproduce

1. Where are you starting? What can you see?
Join Element Call.
Every hour or so there would be an audible notification sound with no visual source.
There is also a notification sound when a participant leaves the call.

### Outcome

#### What did you expect?
In the case of the hidden events, no notification sound.
Participant leaving: Leave call sound.
#### What happened instead?
Default notification sounds.

### Operating system

Linux Mint

### Browser information

Element Nightly version: 2026082401

### URL for webapp

_No response_

### Will you send logs?

Yes

No file, test, or entry point is named. First reproduce the hourly sound and participant-leave sound in Element Call, then trace notification handling for hidden events and call departures; done means hidden events make no sound and a participant leaving uses the intended leave-call sound.

Written by the indexing model from the issue text.