docs(site): default to dark theme

Set colorMode.defaultMode to dark and respectPrefersColorScheme to false so
the docs site loads dark-first regardless of OS preference. The light/dark
toggle remains available.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: shossain786

docs-site/docusaurus.config.js

@@ -45,9 +45,9 @@ const config = {
     ({
       image: 'img/selenium-boot-social.png',
       colorMode: {
-        defaultMode: 'light',
+        defaultMode: 'dark',
         disableSwitch: false,
-        respectPrefersColorScheme: true,
+        respectPrefersColorScheme: false,
       },
       navbar: {
         title: 'Selenium Boot',