Add a shared grouped action rail for Browser, Desktop, and Editor floating surface modals so modality switches, canvas docking, focus mode, New, and close controls appear in a consistent order with separators. Fix the Desktop focus button class collision that hid the canvas docking button, and align dock button labels with canvas terminology.
Add the built-in _desktop plugin as the owner of Xpra/Xfce lifecycle, /desktop route installation, Desktop state, session APIs, surface registration, and the linux-desktop skill. Leave _office with explicit compatibility facades and self-update delegates so users coming from 1.10 through 1.13 keep their runtime cleanup path.
