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>

.htmlvalidate.json

@@ -0,0 +1,8 @@
+{
+  "extends": ["html-validate:recommended"],
+  "rules": {
+    "require-sri": "off",
+    "no-inline-style": "off",
+    "void-style": "off"
+  }
+}