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/utils/seo.ts

@@ -1,4 +1,5 @@
 import type { CollectionEntry } from "astro:content";
+import type { ImageMetadata } from "astro:assets";
 
 export const SITE_URL = "https://mariosant.dev";
 export const TWITTER_HANDLE = "@marios_ant";
@@ -42,6 +43,9 @@ export const deriveDescription = (body: string, max = 155): string => {
 
 type ArticleEntry = CollectionEntry<"articles">;
 
+export const imageSrc = (img: ImageMetadata | string): string =>
+  typeof img === "string" ? img : img.src;
+
 export const personJsonLd = (): string =>
   JSON.stringify({
     "@context": "https://schema.org",
@@ -62,7 +66,7 @@ export const articleJsonLd = (entry: ArticleEntry): string => {
     "@type": "Article",
     headline: entry.data.title,
     description,
-    image: entry.data.coverImage.url,
+    image: imageSrc(entry.data.coverImage.url),
     datePublished: entry.data.date.toISOString(),
     author: {
       "@id": PERSON_ID,