upgrade deps to latest & migrate to astro:assets Image

- bump astro 5→6, @astrojs/mdx 4→6, astro-seo 0.8→1.1, sharp 0.33→0.35, plus minor updates
- move local images from public/ to src/assets/
- replace <img> with <Image> from astro:assets (inferSize for remote URLs)
- content schema uses image() helper for local covers
- eager-load above-the-fold images (article covers, hero avatars)

src/components/home/introduction.astro

@@ -1,16 +1,18 @@
 ---
+import { Image } from "astro:assets";
 import Button from "@/components/button.astro";
 import Link from "@/components/link.astro";
+import mariosant from "@/assets/mariosant.webp";
 ---
 
 <header class="flex flex-col md:gap-12 gap-5 p-4 sm:p-6 lg:p-8 max-w-3xl mx-auto w-full">
   <div class="flex items-center justify-between">
     <a href="/" aria-label="Home">
-      <img
+      <Image
+        src={mariosant}
         alt="Marios Antonoudiou"
-        src="/mariosant.webp"
-        width="64"
-        height="64"
+        width={64}
+        height={64}
         class="w-16 h-16 rounded-full border border-gray-200 dark:border-gray-800 dark:bg-slate-300 block"
         loading="eager"
       />