chore: temporarily disable Tailwind CSS section

- Comment out Tailwind nav links in header and sidebar
- Comment out Tailwind card on landing page
- Remove tailwind from router SECTIONS array
- Remove tailwind from landing page progress tracking

Tailwind content and styles remain in codebase for easy re-enabling.

src/helpers/router.js

@@ -27,7 +27,7 @@ export const RouteType = {
 /**
  * Valid section IDs
  */
-const SECTIONS = ["css", "html", "tailwind", "markdown"];
+const SECTIONS = ["css", "html", "markdown"]; // tailwind temporarily disabled
 
 /**
  * Valid language codes for URL-based switching