feat: integrate WebP/AVIF image optimization into build pipeline

- Add sharp dependency for image format conversion
- Generate WebP (quality 80) and AVIF (quality 70) variants for all raster images
- Render <picture> elements with srcset fallbacks (AVIF > WebP > original)
- SVG images remain as <img> without picture wrapper
- Update Dockerfile to install libvips for sharp, copy from public/ dir
- Add nginx cache rules for .webp and .avif files
- Add .gitignore for node_modules, public, dist

k8s/service.yaml

@@ -1,7 +1,7 @@
 apiVersion: v1
 kind: Service
 metadata:
-  name: signalledger
+  name: news-site
   namespace: openclaw-private
   labels:
     app.kubernetes.io/name: signalledger