A muted player stays muted when you change its volume (#5706)

# What does this implement/fix?

Changing the volume of a muted player silently unmuted it. Muting is
something the user chose, so a volume command should not undo it: the
level set while muted is simply the one the player comes back at when it
is unmuted again.

Volume and mute are now independent for any player that can treat them
that way. Fake mute is the exception, since it is simulated with the
volume itself.

**Related issue (if applicable):**

- related issue <link to issue>

## Types of changes

- [ ] Bugfix (non-breaking change which fixes an issue) — `bugfix`
- [ ] New feature (non-breaking change which adds functionality) —
`new-feature`
- [x] Enhancement to an existing feature — `enhancement`
- [ ] New music/player/metadata/plugin provider — `new-provider`
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected) — `breaking-change`
- [ ] Refactor (no behaviour change) — `refactor`
- [ ] Documentation only — `documentation`
- [ ] Maintenance / chore — `maintenance`
- [ ] CI / workflow change — `ci`
- [ ] Dependencies bump — `dependencies`

Changes:

- a volume command no longer unmutes the player it is sent to
- a muted player in a group keeps its mute through a group volume change
- fake mute still follows the volume, because that is how it is
simulated

## Checklist

- [x] The code change is tested and works locally.
- [x] `pre-commit run --all-files` passes.
- [x] `pytest` passes, and tests have been added/updated under `tests/`
where applicable.
- [ ] For changes to shared models, the companion PR in
`music-assistant/models` is linked.
- [ ] For changes affecting the UI, the companion PR in
`music-assistant/frontend` is linked.
- [x] I have read and complied with the project's [AI
Policy](https://github.com/music-assistant/.github/blob/main/AI_POLICY.md)
for any AI-assisted contributions.
- [ ] I have [raised a PR against the documentation
repository](https://github.com/music-assistant/music-assistant.io/blob/main/CONTRIBUTING.md)
targeting the main or beta branch as appropriate.