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>

astro.config.mjs

@@ -3,6 +3,7 @@ import { defineConfig } from "astro/config";
 import starlight from "@astrojs/starlight";
 import sitemap from "@astrojs/sitemap";
 import remarkSmartypants from "remark-smartypants";
+import { unified } from "@astrojs/markdown-remark";
 
 // https://astro.build/config
 export default defineConfig({
@@ -53,18 +54,18 @@ export default defineConfig({
         {
           label: "ISPmail for Debian 13",
           // slug: "ispmail-trixie",
-          autogenerate: { directory: "ispmail-trixie" },
+          items: [{ autogenerate: { directory: "ispmail-trixie" } }],
         },
           /*
         {
           label: "ISPmail for Debian 12",
           // slug: "ispmail-bookworm",
-          autogenerate: { directory: "ispmail-bookworm" },
+          items: [{ autogenerate: { directory: "ispmail-bookworm" } }],
         },
         */
         {
           label: "Misc articles",
-          autogenerate: { directory: "articles" },
+          items: [{ autogenerate: { directory: "articles" } }],
         },
       ],
       logo: {
@@ -77,10 +78,12 @@ export default defineConfig({
   ],
 
   markdown: {
-    remarkPlugins: [
-      // remove the substitution of -- to –
-      // @ts-ignore
-      [remarkSmartypants, { dashes: false }],
-    ],
+    processor: unified({
+      remarkPlugins: [
+        // remove the substitution of -- to –
+        // @ts-ignore
+        [remarkSmartypants, { dashes: false }],
+      ],
+    }),
   },
 });