fix(docs-infra): keep an explicit theme choice when the OS scheme changes

`watchPreferredColorScheme` applied the OS scheme unconditionally, while
`setTheme`, `loadThemePreference` and the bootstrap script in index.html all
only follow it for `auto`. Choosing Light and then letting the OS switch to
dark repainted the site dark while the theme menu still reported Light, until
a reload restored it.

adev/src/app/core/services/theme-manager.service.ts

@@ -77,6 +77,9 @@ export class ThemeManager {
 
   private watchPreferredColorScheme() {
     window.matchMedia(PREFERS_COLOR_SCHEME_DARK).addEventListener('change', (event) => {
+      if (this.theme() !== 'auto') {
+        return;
+      }
       const preferredScheme = event.matches ? 'dark' : 'light';
       this.setThemeBodyClasses(preferredScheme);
     });