Upgrade to Astro 6.1.9 and migrate to content layer API

Moves content config from src/content/config.ts to src/content.config.ts
with glob loaders as required by Astro 6. Replaces entry.slug with entry.id
and entry.render() with render(entry) throughout. Removes legacy collections
option from astro.config.mjs.

src/pages/index.astro

@@ -25,7 +25,7 @@ const entries = (await getCollection('entries'))
           {entries.map(entry => (
             <li>
               <a
-                href={`/entries/${entry.slug}/`}
+                href={`/entries/${entry.id}/`}
                 class="flex items-center gap-2 px-3 py-2 rounded-lg hover:bg-zinc-800 transition-colors group"
                 style="background: var(--bg-card)"
               >