feat: add legal tabs and update header assets

Switch the legal page to tabbed EULA/privacy content with URL sync, update header styling, and migrate footer icons to Ant Design.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/markdown.d.ts

@@ -0,0 +1,6 @@
+declare module "*.md" {
+  const attributes: Record<string, any>
+  const html: string
+  const toc: { level: string; content: string; slug: string }[]
+  export { attributes, html, toc }
+}