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

public/llms.txt

@@ -10,7 +10,7 @@ Public sections:
 - /focus-areas/: Core focus areas.
 - /about-us/: Firm vision, mission, values, and approach.
 - /contact/: Contact form, phone, and email.
-- /insights/: Prepared future publishing section.
+- /insights/: Articles and analysis (coming soon).
 
 This file is supplemental. The complete public content is available in crawlable HTML pages with
 metadata and structured data.