The functional redesign never touched pluri_bottom_navigation.dart, so the
bar kept the old glass + magenta/coral language while every other surface
moved on. Rebuilds it to prototype t4/4a: a 52px opaque #0A1B24 pill with a
110x74 balloon raised over the active tab, a teal radial glow behind it, the
active item lifted 15px with its label, and inactive items reduced to a 23px
icon at 46% opacity.
The five tabs and their PluriIconGlyph icons are unchanged, per the standing
decision — only the bar's shape, colour and behaviour move.
altura and PluriLayout.bottomChromeInset are both asserted against the real
laid-out height rather than hardcoded guesses.
