chore: upgrade to Astro v6

- Upgrade astro from 5.18.2 to 6.4.4
- Upgrade @astrojs/starlight from 0.35.2 to 0.39.3
- Upgrade @astrojs/sitemap from 3.5.0 to 3.7.3
- Upgrade @astrojs/check from 0.9.4 to 0.9.9
- Upgrade typescript from 5.9.3 to 6.0.3
- Upgrade sharp from 0.32.6 to 0.34.5
- Fix Starlight sidebar config for v0.39.0 (autogenerate → items array)
- Migrate markdown plugins to unified() API

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

package.json

@@ -12,14 +12,14 @@
     "astro": "astro"
   },
   "dependencies": {
-    "@astrojs/check": "^0.9.4",
-    "@astrojs/sitemap": "^3.5.0",
-    "@astrojs/starlight": "^0.35.2",
+    "@astrojs/check": "^0.9.9",
+    "@astrojs/sitemap": "^3.7.3",
+    "@astrojs/starlight": "^0.39.3",
     "@splidejs/splide": "^4.1.4",
-    "astro": "^5.13.2",
-    "astro-splide": "^2.0.0",
-    "sharp": "^0.32.6",
-    "typescript": "^5.9.2"
+    "astro": "^6.4.4",
+    "astro-splide": "^2.1.1",
+    "sharp": "^0.34.5",
+    "typescript": "^6.0.3"
   },
   "engines": {
     "bun": ">=1.3"