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)

package.json

@@ -11,20 +11,20 @@
     "astro": "astro"
   },
   "dependencies": {
-    "@astrojs/mdx": "^4.2.0",
-    "@astrojs/rss": "^4.0.11",
-    "@astrojs/sitemap": "^3.2.1",
-    "@fontsource-variable/inter": "^5.2.5",
-    "@iconify-json/heroicons": "^1.2.1",
+    "@astrojs/mdx": "^6.0.3",
+    "@astrojs/rss": "^4.0.18",
+    "@astrojs/sitemap": "^3.7.3",
+    "@fontsource-variable/inter": "^5.2.8",
+    "@iconify-json/heroicons": "^1.2.3",
     "@iconify-json/hugeicons": "^1.2.29",
-    "@iconify-json/mynaui": "^1.2.5",
-    "@iconify-json/simple-icons": "^1.2.28",
-    "@tailwindcss/typography": "^0.5.16",
-    "@tailwindcss/vite": "^4.0.14",
-    "astro": "^5.5.2",
+    "@iconify-json/mynaui": "^1.2.17",
+    "@iconify-json/simple-icons": "^1.2.86",
+    "@tailwindcss/typography": "^0.5.20",
+    "@tailwindcss/vite": "^4.3.1",
+    "astro": "^6.4.7",
     "astro-icon": "^1.1.5",
-    "astro-seo": "^0.8.4",
-    "sharp": "^0.33.5",
-    "tailwindcss": "^4.0.14"
+    "astro-seo": "^1.1.0",
+    "sharp": "^0.35.1",
+    "tailwindcss": "^4.3.1"
   }
 }