Deprecate `global_theme_preference_switch` in favor of `global_theme_preference_buttons` (#8505)

`global_theme_preference_switch` can only toggle between dark and light
mode, with no way back to following the system theme.

`global_theme_preference_buttons` covers all three, and now uses compact
icon-only buttons (💻 🌙 ☀), so it fits in a top bar too. The demo app now
uses it.

* Rename `ThemePreference::radio_buttons` to `ThemePreference::buttons`
(old name deprecated)
* Add `ThemePreference::icon` and `ThemePreference::name`

* [x] I have followed the instructions in the PR template

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>