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)

astro.config.mjs

@@ -7,6 +7,14 @@ import icon from "astro-icon";
 export default defineConfig({
   site: "https://mariosant.dev",
   integrations: [mdx(), sitemap(), icon()],
+  image: {
+    domains: [
+      "images.unsplash.com",
+      "app.ghostwriter.rocks",
+      "990141.apps.zdusercontent.com",
+      "cdn.livechat-files.com",
+    ],
+  },
   vite: {
     plugins: [tailwindcss()],
   },