Pure member movement into MainViewModel.Account.cs (zero behavior change):
session restore (LoadSavedSessionAsync), sign-in/out/change (SignInAsync,
SignOutYouTubeAsync, ChangeAccountAsync), IsConnected + account avatar/name +
AccountStatusLightToolTip. Partial carries its own usings (ytLive.Services for
the INotificationService default interface methods). docs in same commit:
ViewModels/index.md split table + tagging convention.
