refactor(css): extract inline OKLCH values into CSS custom properties

Add 23 design tokens to :root for colors previously hardcoded inline:
hero text (kicker, proof), hero background gradient, hero button gradient,
accent underline (shared across 3 rules), page background gradient,
table row states (hover, focus, open), sticky header bg, search shadows
and focus ring, tag states, and CTA background.

Decorative one-off values (hero noise, sheen, grid overlay) left inline
since they are tightly coupled to their visual effects.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>