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

astro.config.mjs

@@ -1,6 +1,7 @@
 // Static Astro site for BlueCap Strategies. CI: .gitea/workflows/ci.yml (Gitea Actions).
 import { defineConfig } from "astro/config";
 import sitemap from "@astrojs/sitemap";
+import { rehypeResponsiveImages } from "./src/utils/images.mjs";
 
 const site = process.env.PUBLIC_SITE_URL ?? "https://www.bluecapstrategies.com";
 
@@ -8,6 +9,10 @@ export default defineConfig({
   site,
   output: "static",
   trailingSlash: "always",
+  markdown: {
+    // Article column is ~880px wide on desktop, full width below the 960px breakpoint.
+    rehypePlugins: [[rehypeResponsiveImages, { sizes: "(max-width: 960px) 100vw, 880px" }]],
+  },
   integrations: [
     sitemap({
       filter: (page) => !page.includes("/thank-you/"),