feat(code-demo): unify visual theme with MIDI demo

- Match :root variables (--bg, --card, --card2, --border, --muted, --gold) to MIDI demo values so both pages share the same dark base - Add gradient text to .nav-title (white→accent), matching MIDI's treatment - Change nav-tag and nav-badge border-radius to 20px (pill shape) - Add tinted background fill to .nav-badge (consistent with MIDI badge) - Upgrade nav-tag.green/.gold backgrounds to match MIDI pill pattern - Move demo links from nav-right to the left (logo → separator → links → center → badge), matching MIDI's three-zone nav layout exactly - Switch nav from position:fixed to position:sticky; drop body padding-top - Match nav backdrop: rgba(13,17,24,0.92) instead of rgba(9,13,22,.92)