Add CI lint configs (html-validate, linkinator), fix nav a11y and URL routing

- .htmlvalidate.json with relaxed rules for design-system inline styles
- linkinator.config.json skipping mail, external auth/platform origins
- Fix lint:html npm script quoting for Windows-shell compatibility
- Switch astro build.format to 'directory' so /pricing resolves without MultiViews
- trailingSlash: 'ignore' lets both /pricing and /pricing/ work naturally
- Add aria-label to both <nav> landmarks (Primary, Footer) to satisfy html-validate

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

linkinator.config.json

@@ -0,0 +1,13 @@
+{
+  "recurse": true,
+  "silent": true,
+  "skip": [
+    "^https://auth\\.cameleer\\.io",
+    "^https://platform\\.cameleer\\.io",
+    "^https://www\\.cameleer\\.io",
+    "^mailto:",
+    "^https://ec\\.europa\\.eu"
+  ],
+  "retry": true,
+  "concurrency": 10
+}