Clarify the global Autoplay and crossfade default toggles (#6187)

# What does this implement/fix?

The two checkboxes we recently added to the global queue settings were
labelled just "Autoplay" and "Crossfade", which doesn't tell you what
ticking them does (both set the default that every queue starts with).
The labels now say that, and the descriptions no longer repeat it.

- rename `autoplay_enabled` to "Enable Autoplay by default" and
`crossfade_enabled` to "Enable crossfade by default"
- drop the now-redundant first sentence from both descriptions, keeping
the part that matters: the player controls can still toggle it per queue

**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.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5 <[email protected]>
(cherry picked from commit 148bbaa70dd4756777206a61c41451cb07b5c4ec)