refactor: Clean up CSS structure and separate print.css

- Delete orphaned CSS files (color-theme.css, logo-toggle.css,
  skeleton.css, main.new.css) - replaced by modular equivalents
- Delete 08-contexts/_print.css - wrongly created during modularization
- Remove 08-contexts folder (now empty)
- Add print.css as standalone with media="print" in HTML
- Update stale comments referencing old file names
- Remove _print.css import from main.css

print.css remains standalone and will NOT be bundled, as it's a
special case loaded only when printing (media="print").

static/css/main.css

@@ -41,5 +41,4 @@
 /* 06 - Effects */
 @import './06-effects/_skeleton.css';
 
-/* 08 - Contexts */
-@import './08-contexts/_print.css';
+/* NOTE: print.css is loaded separately in HTML with media="print" */