Align Demo navigation hover and keyboard focus with AlvenX

[AlbertXXuu]

The Demo header's ReproLock link now stays transparent at rest and shows the shared glass treatment on hover or keyboard focus. The 35% white fill and 82% optical edge match the AlvenX navigation treatment; a reserved border prevents interaction-time layout shifts, and the independent keyboard focus ring remains visible.

This changes only the Demo's own CSS. Reduced-motion mode disables the transition. Shared assets, evidence, experiments, and product behavior remain unchanged.

Validation:

• Standalone brand verification and single-file Biome formatting passed.
• Actual header, stylesheet, wordmark, and font resources checked in headless Chromium at 390, 900, and 1440 px, with both motion preferences: all six combinations passed idle/hover/focus colors, 44 px target height, visible keyboard focus, and unchanged geometry between states.
• git diff --check passed.
• A sandboxed local pnpm check passed formatting, lint, brand, and type checks, then encountered denied Windows Get-CimInstance access in process tests. It was stopped and no matching test child processes remained. The existing PR CI provides the full gate.