refactor: rename SelectionBarControl.CommandParameter to Selection

[Copilot]

SelectionBarControl.CommandParameter no longer represented an arbitrary command parameter; it consistently carried the active selection model. This change renames the control API to Selection and updates the consumers so the selection flow remains explicit at every binding site.

• Control API

  • Rename the dependency property from CommandParameter to Selection in SelectionBarControl.
  • Update the control's public surface and XML docs to describe the property as the active selection object rather than a generic command argument.

• Internal control bindings

  • Rebind the control's action buttons to pass Selection into play, play-next, add-to-queue, and remove commands.
  • Update the add-to-playlist flyout behavior to receive Selection as its data context.

• Consumer updates

  • Replace CommandParameter="{x:Bind ViewModel.Selection}" with Selection="{x:Bind ViewModel.Selection}" at all SelectionBarControl use sites, including Home and Play Queue surfaces.

<controls:SelectionBarControl
    PlayButtonCommand="{x:Bind ViewModel.PlaySelectedCommand}"
    PlayNextButtonCommand="{x:Bind ViewModel.PlaySelectedNextCommand}"
    RemoveButtonCommand="{x:Bind ViewModel.RemoveSelectedCommand}"
    Selection="{x:Bind ViewModel.Selection}" />

[United600]

It feels unnecessary, so we should drop it from any buttons that don't require it and keep it solely on the AddToPlaylistButton. Unless we revert the commands to require a parameter again, but that doesn't seem practical.