Fill the screen in home-screen (standalone) mode on iOS (#25)

Full-screen views sized every layer against 100dvh. Measured on iOS 18.7 in
a home-screen app (black-translucent status bar, viewport-fit=cover): the
layout viewport that fixed boxes, percentages, dvh and svh resolve against is
the screen minus the status-bar height (873 of 932 points), anchored to the
top, and anything laid out below it is cropped. The fixed shell therefore
stopped one status bar short of the bottom edge and left a blank strip under
the terminal keys.

Let flex size the header and body inside the shell so no descendant needs a
viewport unit, and in standalone mode keep the shell in flow at 100vh, the one
unit that spans the screen there, locking the document instead. Browser tabs
keep the fixed shell, which tracks Safari's collapsing toolbars.


Claude-Session: https://claude.ai/code/session_01NRdayBsqfSrkrJQy4X9Z2v

Co-authored-by: magneto <magneto.core.shadow@gmail.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>