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/realm_logo.js

@@ -99,3 +99,16 @@ export function render() {
         $night_file_input,
     );
 }
+
+export function initialize() {
+    // render once
+    render();
+
+    // Rerender the realm-logo when the browser detects color scheme changes.
+    const dark_mode_media_query_list = window.matchMedia("(prefers-color-scheme: dark)");
+    dark_mode_media_query_list.addEventListener("change", () => {
+        if ($("body").hasClass("color-scheme-automatic")) {
+            render();
+        }
+    });
+}

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();