Eliminates fragile ad-hoc DOM toggling that allowed two mobile screens to
render simultaneously. All transitions now go through setMobileState()
reducer; renderMobile() is the only function that writes
.mobile-screen-active. assertSingleActiveScreen() self-heals violations
and traces them. Test asserts the architectural invariant in source.
Also: DB session lookup fails closed on transient errors instead of
crashing; mobile back-save uses formHash (not UI badge); single-screen
CSS via display:none/flex (no transforms); resize debounced without
reload; aria-hidden warning fix.