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").

templates/index.html

@@ -111,6 +111,8 @@
 
     <!-- CSS - Modular structure with native @import -->
     <link rel="stylesheet" href="/static/css/main.css">
+    <!-- Print styles - loaded separately, only applied when printing -->
+    <link rel="stylesheet" href="/static/css/print.css" media="print">
 
     <!-- Fonts with Preload -->
     <link rel="preconnect" href="https://fonts.googleapis.com">