dark_theme: Re-render realm-logo on prefers-color-scheme changes.

We already re-rendered the realm-logo when the user selects either
the light or dark color scheme, but on `color-scheme-automatic`,
we didn't have any event handlers to trigger a render. This commit
adds a handler to listen for changes to `prefers-color-scheme` property.

Fixes #23548.

static/js/ui_init.js

@@ -609,8 +609,8 @@ export function initialize_everything() {
     settings.initialize();
     initialize_navbar();
     initialize_message_feed_errors();
-    realm_logo.render();
 
+    realm_logo.initialize();
     message_lists.initialize();
     alert_words.initialize(alert_words_params);
     emojisets.initialize();