feat: add light/dark theme to exchange instances with hub SSO sync

Co-authored-by: Cursor <cursoragent@cursor.com>

manual_trading_hub/static/theme.js

@@ -15,6 +15,15 @@
     }
   }
 
+  function broadcastThemeToInstances() {
+    const msg = { type: "hub-theme-sync", theme: get() };
+    document.querySelectorAll("iframe#instance-frame, iframe.instance-frame").forEach((frame) => {
+      try {
+        if (frame.contentWindow) frame.contentWindow.postMessage(msg, "*");
+      } catch (_) {}
+    });
+  }
+
   function apply(theme) {
     const t = normalize(theme);
     const root = document.documentElement;
@@ -26,6 +35,7 @@
     if (meta) meta.setAttribute("content", META[t]);
     root.style.colorScheme = t;
     document.dispatchEvent(new CustomEvent("hub-theme-change", { detail: { theme: t } }));
+    broadcastThemeToInstances();
     return t;
   }