Removes the bubble_layout toggle from Settings, all persistence, CSS, i18n strings, and the UI docs demo. The CSS was already effectively dead. Users with a saved bubble_layout value in settings.json get a clean migration via _SETTINGS_LEGACY_DROP_KEYS.
Credit: @aronprins (PR #760 / #777)
Co-authored-by: aronprins <aronprins@users.noreply.github.com>