Add postMessage listener for SGIWorld theme sync

Listen for 'sgiworld-theme' messages from parent window
and apply the corresponding theme (dark/white).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: WenlongZhang0517

static/app.js

@@ -90,6 +90,14 @@ const API = '';
     document.body.appendChild(sw);
     apply(saved);
   });
+
+  // Listen for theme changes from parent SGIWorld
+  window.addEventListener('message', function (event) {
+    if (event.data && event.data.type === 'sgiworld-theme') {
+      var theme = event.data.theme === 'dark' ? 'dark' : 'white';
+      apply(theme);
+    }
+  });
 })();
 
 /* ── State ───────────────────────────────────────────────────────────── */