Extends `SettingsKey.isPlatformSpecific` to cover home, tab bar, and
sidebar layout, plus player details panel and video swipe actions, so
iOS devices sync these with other iOS devices (and tvOS with tvOS,
macOS with macOS) instead of overwriting each other via the shared
iCloud key. Adds a one-shot migration that copies legacy unprefixed
values into the new platform-prefixed slots locally and in iCloud,
preserving protected-key timestamps.
