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

package-lock.json

@@ -20,6 +20,7 @@
         "@astrojs/check": "^0.9.4",
         "@playwright/test": "^1.60.0",
         "prettier": "^3.5.3",
+        "sharp": "0.34.5",
         "typescript": "^5.8.3"
       }
     },
@@ -773,8 +774,8 @@
       "version": "1.1.0",
       "resolved": "https://registry.npmjs.org/@img/colour/-/colour-1.1.0.tgz",
       "integrity": "sha512-Td76q7j57o/tLVdgS746cYARfSyxk8iEfRxewL9h4OMzYhbW4TAcppl0mT4eyqXddh6L/jwoM75mo7ixa/pCeQ==",
+      "devOptional": true,
       "license": "MIT",
-      "optional": true,
       "engines": {
         "node": ">=18"
       }
@@ -2626,8 +2627,8 @@
       "version": "2.1.2",
       "resolved": "https://registry.npmjs.org/detect-libc/-/detect-libc-2.1.2.tgz",
       "integrity": "sha512-Btj2BOOO83o3WyH59e8MgXsxEQVcarkUOpEYrubB0urwnN10yQ364rsiByU11nZlqWYZm05i/of7io4mzihBtQ==",
+      "devOptional": true,
       "license": "Apache-2.0",
-      "optional": true,
       "engines": {
         "node": ">=8"
       }
@@ -4952,9 +4953,9 @@
       "version": "0.34.5",
       "resolved": "https://registry.npmjs.org/sharp/-/sharp-0.34.5.tgz",
       "integrity": "sha512-Ou9I5Ft9WNcCbXrU9cMgPBcCK8LiwLqcbywW3t4oDV37n1pzpuNLsYiAV8eODnjbtQlSDwZ2cUEeQz4E54Hltg==",
+      "devOptional": true,
       "hasInstallScript": true,
       "license": "Apache-2.0",
-      "optional": true,
       "dependencies": {
         "@img/colour": "^1.0.0",
         "detect-libc": "^2.1.2",