siso: webui restructure app shell css

Make the header sticky and give it a known fixed height that the navrail
can position itself relative to.

Do this instead of fixing the app shell and setting `overflow: hidden`
on the body then needing to set `overflow: scroll-y` on the inner page
content, such that we can rely on the browser's native understanding of
where the scrollbar should be when navigating back and forth + not break
browsers that try to be fancy with collapsing their chrome on scroll

While we're at it, add a right margin so that we're actually doing a MD3
style "card" as the inner page content.

Change-Id: I0858991e7c46b7ddde9c9fe2d67a181c6a6a6964
Reviewed-on: https://chromium-review.googlesource.com/c/build/+/8131407
Reviewed-by: Marc Jin <jmarc@google.com>
Commit-Queue: Richard Wang <richardwa@google.com>