fix: retry theme icon update until bricks button element exists

wwwroot/shell_theme.js

@@ -122,6 +122,18 @@
     initRouter();
   }
 
+  // Bricks widgets render asynchronously after DOMContentLoaded.
+  // Retry updateThemeIcon until the button element exists.
+  (function retryThemeIcon() {
+    var btn = document.getElementById('theme_toggle_btn');
+    if (btn) {
+      var theme = document.documentElement.getAttribute('data-theme') || 'dark';
+      updateThemeIcon(theme);
+    } else {
+      setTimeout(retryThemeIcon, 200);
+    }
+  })();
+
   // Expose global functions for bricks bind access
   window.sageToggleTheme = toggleTheme;
   window.sageToggleSidebar = toggleSidebar;