On touch-primary devices (iPad Safari) the on-screen keyboard shrinks the
visual viewport but not the layout viewport, so the bottom-pinned composer
sits behind the keyboard. Compute a --keyboard-bottom-inset from visualViewport
and pad the composer up by it; clear it when the keyboard closes / on
non-touch / no-visualViewport. Reuses the existing visualViewport + pageshow
hooks; no-op on desktop/fine-pointer.
Co-authored-by: nesquena-hermes <nesquena-hermes@users.noreply.github.com>