Save per user volume adjustment

### Your use case

#### What would you like to do?
I would like that the volume I set a call participant to is used across all future calls in the same room as well as other rooms.

#### Why would you like to do it?
Because some people have their microphone set in a way that is too loud and I have to make them more quite every time I join a call.

#### How would you like to achieve it?
This does not require any additional UI, the volume bar of an individual user would be set to the value I set it in previous call.

### Have you considered any alternatives?

I am not aware of any.

### Additional context

_No response_

No files, tests, or entry points are named in the issue. Start by locating where per-participant volume adjustments are held and where call or room state is persisted, then trace how those values are applied when joining a call. Done means a participant's volume adjustment is reused in future calls across rooms without adding UI.

Written by the indexing model from the issue text.