Add the navigation rail, and finish the desktop screen

A navigation rail is a new part: 80dp wide, full height, its destinations
stacked below a 44dp inset, with the same tabs as the navigation bar and
left / right corner radii. It stands on whichever edge of the screen it is
nearer to; tidy keeps it there and lays the rest of the screen out in the
body beside it.

Switching a screen across the M3 expanded width (840dp) swaps the bar and
the rail: the first navigation bar standing on its own becomes a rail on a
desktop screen, and a rail becomes a bar again on a phone. Parts keep
their place measured from the body's left edge, bars take the body's
width, and a bar or a tall part dropped on a desktop screen lands beside
the rail and within the screen's height. A part inside a hand-made group
is left alone.

Desktop details: a top app bar wider than a phone has no status-bar inset;
parts dropped on a desktop keep their phone size, with the phone column
offered as a width preset; the prompt writes the rail first on its own
line, drops the system-inset notes and the wallpaper wording on the web,
and the README describes both screen sizes in three languages.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>