...to thwart shadow dom and inheritance issues

- Allows setting activated state on button

- Hide menu button when sidebar is shown
- Use close button for sidebar close
- Introduce `.fade` utility class
- Restructure menu items to bring more of it "in house" and under the
  direct control of os-scene

Introduce ResponsiveElement helper class which uses a resize observer to
generate breakpoint information about the container context. This is to
work around the fact that container queries are currently broken for
slotted content. The spec is updated and browsers are fixing it, but
it's going to be a bit before there is broad support for slotted
container queries.

See https://chromestatus.com/feature/5242724333387776

Navstack pushes groups of panels without any animation, but dismisses
them with a back-stack-like slide-off-of-screen animation.

- Introduce animation utils
- Remove os-toolbar, move .toolbar to shared styles. We ended up needing
  this flexibility.
- Introduce navpanel as a scrollable content view with a toolbar.

...also add AI icon placeholder.

Splits sidebar 50/50.

Switched from ResponsiveElement to container queries for sidebar. This
works here because we aren't cuing off of slotted content. Allows us to
remove the workaround for the first render animation glitch. Simpler all
around.

And fix width at 756, which I think should be enough for 80 columns in
the editor. If not, we'll adjust.

...and fix shadow var.