perf: responsive AVIF/WebP images; a11y contrast and link-text fixes

- scripts/optimize-images.mjs generates hashed AVIF/WebP variants at build time (sharp)
- Picture.astro + rehype plugin serve them via <picture> with srcset/sizes; hero gets fetchpriority=high
- footer CTA and active service-nav item meet WCAG AA contrast
- homepage Learn More link gets screen-reader-only context
- llms.txt: insights described as coming soon

.gitignore

@@ -2,6 +2,9 @@
 node_modules/
 dist/
 .astro/
+# Generated by scripts/optimize-images.mjs at build time
+public/assets/opt/
+src/data/image-manifest.json
 
 # Secrets — only .env.example is committed
 .env