Function nav_active 

pub fn nav_active() -> Style

Active tab marker for the top navigation bar.

Modern terminals (Ghostty, iTerm2 3.4+, WezTerm, Kitty, recent VTE): bold white label with a brand-coloured underline via SGR 58. Letters stay white so the active tab reads as a label, not as a second brand splash.

Legacy terminals without SGR 58 (macOS Terminal): bold white label with the basic SGR 4 underline only. The underline inherits the foreground (white) because Terminal.app cannot decouple underline colour from fg. Brand accent on the underline is sacrificed; the label stays white so it does not visually compete with the solid purple brand badge.