fix(dev): collapse the docs rail, drop its scrollbar, highlight code

[vreshch]

Three UX complaints on the deployed docs site (ds.agentage.io). dev/ + e2e/ only — no src/components changes.

1. Double scrollbar on the left

The rail nested two scroll containers: the shell <aside> (overflow-y-auto) around DocSidebar's own sticky max-h-screen overflow-y-auto inner div. Both painted a bar from the global ::-webkit-scrollbar styling in base.css, right next to the page bar.

• The inner scroller is flattened (scoped to .docs-rail, so the DocSidebar demo keeps its own).
• The rail gets scrollbar-width: thin + a transparent thumb that only appears on hover/focus-within, with matching ::-webkit-scrollbar overrides since Chromium prefers the pseudo-elements once they exist.

2. Left navigation is not collapsed

DocsNav (new, dev/components/docs-nav.tsx) dogfoods the DS Collapsible compound API:

• One group per category, collapsed by default, chevron indicator, aria-expanded/aria-controls for free.
• The group holding the current route auto-expands - on deep links too - and the active item is scrolled into the rail (rail scrollTop only, never the page).
• Open groups persist for the session in sessionStorage.

With groups collapsed the rail fits the viewport, which is what mostly removes complaint 1; the thin-scrollbar fix covers the long-group case.

3. Code is not highlighted

highlight.js as a devDependency (dev/ is not published - the library's dependencies are untouched), four languages registered, fed through the DS CodeBlock children slot so the chrome stays DS. All import snippets, the getting-started and theming snippets, and the CodeBlock demo are highlighted.

The theme maps hljs classes to semantic tokens only. Every one clears 4.5:1 on --color-card in both themes (dark / light): muted-foreground 4.66 / 4.76, primary-emphasis 9.32 / 5.45, success 8.41 / 5.43, warning 6.82 / 5.43, info 5.21 / 5.47, destructive 4.91 / 5.67.

Verification

• npm run verify green (434 unit tests).
• Playwright green in mcr.microsoft.com/playwright:v1.62.1-noble, 43/43 - the axe WCAG A/AA sweep passes over all 8 categories, every component page and /, /install, /theming in both themes with the new highlight colours.
• New specs: deep-link expands only the route's group, keyboard toggle + session persistence, "the rail is the only scroller in the aside", and highlighting in both themes.
• Baselines regenerated in the container: component-page and category-index only (the two full-page shots that include the rail).
• Browser (:5182): exactly one visible scrollbar when several categories are open and none when the rail fits, groups collapse/expand, active item auto-scrolled on a cold deep link, code visibly multi-coloured in dark and light, no console errors.