refactor : merge light-mode into script.js, rename panel to side-panel, fix print styles

Integrate light-mode.js into script.js, unify .panel/.side-panel classes
into .side-panel, scope theme variables to @media screen for proper
print light mode, and add page-break after chapo.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

assets/css/src/_print.scss

@@ -1,5 +1,5 @@
 @media print {
-  .panel,
+  .side-panel,
   nav,
   .toggle-light {
     display: none;