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>

package.json

@@ -11,7 +11,7 @@
     "astro": "astro",
     "test": "vitest run",
     "test:watch": "vitest",
-    "lint:html": "html-validate 'dist/**/*.html'",
+    "lint:html": "html-validate \"dist/**/*.html\"",
     "lint:links": "linkinator dist --recurse --silent",
     "lh": "lhci autorun"
   },