Give the shell one header, a skip link, and a cookie-aware first paint.

Breakpoint clones were still in the tab order, marketing always painted Log in, and at 430px the logo overlay covered Use cases and Learn.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/mainContent.ts

@@ -0,0 +1,2 @@
+/** DOM id of the group-layout `<main>` landmark; skip-to-content targets this. */
+export const MAIN_CONTENT_ID = "main-content";