chore(website): upgrade astro 6.4.5 + starlight 0.40.0 (clears markdown deprecation)

Astro 6.4.5 deprecated markdown.remarkPlugins/rehypePlugins/remarkRehype in
favour of the new unified() API. The warning came from @astrojs/starlight
0.39.3's own integration code, not our config; Starlight 0.40.0 adopts the new
API, so it's gone.

- astro 6.4.4 -> 6.4.5; @astrojs/starlight 0.39.3 -> 0.40.0
  (brings astro-expressive-code 0.43.1, @astrojs/markdown-remark 7.2.0)
- Starlight 0.40.0 adds an optional @astrojs/markdown-satteri peer (an opt-in
  high-performance markdown engine); it's an optional peer so pnpm doesn't
  install it, and we have no need for it — we stay on the default
  markdown-remark/unified pipeline

Verified: deprecation gone; pnpm build clean (25 pages); rendering signals
unchanged vs baseline (highlighting, > prompt + copy-button :has() CSS, asides,
embedded casts); pnpm audit clean.

website/package.json

@@ -12,11 +12,11 @@
     "astro": "astro"
   },
   "dependencies": {
-    "@astrojs/starlight": "^0.39.3",
+    "@astrojs/starlight": "^0.40.0",
     "@astrojs/starlight-tailwind": "^5.0.0",
     "@tailwindcss/vite": "^4.3.0",
     "asciinema-player": "^3.15.1",
-    "astro": "^6.3.1",
+    "astro": "^6.4.5",
     "sharp": "^0.34.5",
     "tailwindcss": "^4.3.0"
   }