perf: enable Next.js image optimization across backoffice

Remove `unoptimized` flag from all thumbnail/cover Image components
and add proper responsive `sizes` props. Convert raw `<img>` tags on
the libraries page to next/image. Add 24h minimumCacheTTL for
optimized images. BookPreview keeps `unoptimized` since the API
already returns optimized WebP.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/backoffice/next.config.mjs

@@ -1,7 +1,10 @@
 /** @type {import('next').NextConfig} */
 const nextConfig = {
   output: "standalone",
-  typedRoutes: true
+  typedRoutes: true,
+  images: {
+    minimumCacheTTL: 86400,
+  },
 };
 
 export default nextConfig;