Set a global default for the Autoplay and Crossfade switches (#6130)

# What does this implement/fix?

The per-player Autoplay and Crossfade switches always started out the
same way (Autoplay on, crossfade off) and there was no place to change
that. Settings > Core > Player Queues now has an on/off default for
both. Every player follows the default until you use its own switch;
from that moment the player keeps its own choice and ignores the
default. Changing a default reaches all players you never toggled,
immediately.

- add global Autoplay and Crossfade on/off defaults to the Player Queues
settings
- a player's switch pins a per-queue override; everything else follows
the global value live
- a default change reaches every following player right away; the
audible effect lands on the next track transition, like the other global
queue settings
- existing players adopt the new defaults once; a switch used after the
update sticks as before

**Related issue (if applicable):**

- n/a

## Types of changes

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- [ ] 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`

## 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.

---------

Co-authored-by: Marcel van der Veldt <[email protected]>