Add Theme::var(name, ctx) (CSS custom-property getter) and Theme::apply_to_egui(ctx)
which maps the palette onto egui's global Visuals so standard widgets share the
theme colors. Add a mobile::palette::Palette built from the theme each frame and
replace the duplicated per-file C_* color constants across all mobile modules
(omni, topbar, stack, inspector, transport, intent, shell) with it. Add --scrim
and --accent-* category vars to styles.css. Apply the theme visuals every frame in
update(). Mobile and the main UI now share one CSS-variable-driven palette that
responds to light/dark/user themes.
Skyler Lehmkuhl