Give the hero the gateway graph it is describing #29

The hero is a headline, a paragraph and two buttons on empty space. Everything under it is text as well, so the page never shows what any of these tools do.

Add components/HeroGraph.tsx: a three.js graph with agents on the left, one gateway node in the middle, tools on the right, and pulses running left to right along the edges. That is Selat's flow, so the visual carries information instead of decorating.

What it has to respect:

• Loaded through next/dynamic with ssr: false, inside a fixed height box so nothing shifts on arrival.
• Only three. @react-three/fiber and drei cost several times more for a scene this small.
• Colours read from the design package custom properties, so the theme toggle drives it and no new colour enters the site outside the token system.
• prefers-reduced-motion: reduce draws one static frame and never starts the animation loop.
• The loop stops while the tab is hidden or the canvas is out of view.

The hero becomes two columns from lg up and stays stacked below that.