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

src/components/Picture.astro

@@ -0,0 +1,27 @@
+---
+// <img> for a /public image path, upgraded to <picture> with AVIF/WebP srcsets when
+// scripts/optimize-images.mjs has variants for it. The original stays as the <img> fallback.
+// `sizes` should describe the rendered width so phones fetch a phone-sized file.
+import type { HTMLAttributes } from "astro/types";
+import { imageVariants } from "@utils/images.mjs";
+
+type Props = Omit<HTMLAttributes<"img">, "src" | "srcset" | "sizes"> & {
+  src: string;
+  sizes?: string;
+};
+
+const { src, sizes = "100vw", width, height, ...attrs } = Astro.props;
+const variants = imageVariants(src);
+---
+
+{
+  variants ? (
+    <picture>
+      <source type="image/avif" srcset={variants.avif} sizes={sizes} />
+      <source type="image/webp" srcset={variants.webp} sizes={sizes} />
+      <img src={src} width={width ?? variants.width} height={height ?? variants.height} decoding="async" {...attrs} />
+    </picture>
+  ) : (
+    <img src={src} width={width} height={height} {...attrs} />
+  )
+}