feat(docs-site): brand the docs to match the punktfunk website

Theme the Fumadocs docs site with the punktfunk identity, mirroring the
marketing site:

- Swap the stock `neutral` preset for `purple`, then override --color-fd-*
  with the violet lens-mark palette (#6c5bf3 / #a79ff8). The brand is the
  violet, not the site's blue marketing background, so the blue is not used
  as a reading surface; dark mode tints the chrome toward the app-icon
  violet-dark (#1c1530).
- Adopt @unom/ui's token contract (--brand/--primary/--accent + bg-brand
  etc.) as the shared token source, and @source its dist.
- Load Geist (the brand typeface) via @fontsource-variable/geist.
- Add the BrandMark lens to the nav + landing hero, wire the brand
  favicon.svg, and add Docs/Website nav links.
- Keep the Fumadocs light/dark toggle.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs-site/src/lib/layout.shared.tsx

@@ -1,10 +1,21 @@
 import type { BaseLayoutProps } from 'fumadocs-ui/layouts/shared'
+import BrandMark from '@/components/BrandMark'
 
 // Shared chrome (nav title, links) for both the docs layout and the home layout.
+// The lens mark + wordmark mirror the punktfunk marketing site's header.
 export function baseOptions(): BaseLayoutProps {
   return {
     nav: {
-      title: 'punktfunk',
+      title: (
+        <>
+          <BrandMark className="size-6" />
+          <span className="font-display font-semibold tracking-tight">punktfunk</span>
+        </>
+      ),
     },
+    links: [
+      { text: 'Docs', url: '/docs' },
+      { text: 'Website', url: 'https://punktfunk.unom.io' },
+    ],
   }
 }