Let speakers with guest access play without a setup step (#6088)

# What does this implement/fix?

A speaker that allows guest access can already play, but Music Assistant
still marked it "setup
required" and made you click through an approval screen first. That is
inconsistent: an older
unencrypted speaker needs no approval at all, even though it is the less
secure of the two.

Guest-capable devices are now approved automatically when they connect,
so they show up ready to
play. Pairing stays available as an optional upgrade through
Reconfigure.

- a device offering guest access is approved on connect, so it plays
without any setup step
- devices with an audio input still show the approval screen once, since
that choice cannot be
  split out of the setup state
- the approval screen is simpler, and its button now reads Finish
instead of Next
- the "Require pairing" action is gone, along with the enable/disable
unpaired access controls in
device management: whether a device accepts unpaired connections is the
device's own setting
- a paired-free guest device now shows a plain note instead of a warning
- Reconfigure is hidden on devices with nothing left to set up, instead
of opening a flow that can
  only abort

## 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`

## 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.