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/package.json

@@ -10,8 +10,11 @@
     "lint": "tsc --noEmit"
   },
   "dependencies": {
+    "@fontsource-variable/geist": "^5.2.9",
     "@tanstack/react-router": "^1.121.0",
     "@tanstack/react-start": "^1.121.0",
+    "@unom/style": "^0.4.4",
+    "@unom/ui": "^0.8.16",
     "fumadocs-core": "^16.10.1",
     "fumadocs-ui": "^16.10.1",
     "react": "^19.0.0",