fix(embedded): move dataMask store.subscribe into an effect

Per codeant review on #39893: subscribing to the Redux store from inside
the render body of EmbededLazyDashboardPage registered a new listener on
every render with no cleanup, leaking subscriptions and double-emitting
observeDataMask events on each store update. StrictMode's dev-mode
double-mount made the leak immediately visible.

Move the subscription into a useEffect keyed on emitDataMasks, returning
the Redux unsubscribe so React tears the listener down on unmount.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

superset-frontend/src/embedded/index.tsx

@@ -18,7 +18,7 @@
  */
 import 'src/public-path';
 
-import { lazy, StrictMode, Suspense } from 'react';
+import { lazy, StrictMode, Suspense, useEffect } from 'react';
 import { createRoot, type Root } from 'react-dom/client';
 import { BrowserRouter as Router, Route } from 'react-router-dom';
 import { Global } from '@emotion/react';
@@ -68,18 +68,19 @@ const LazyDashboardPage = lazy(
 
 const EmbededLazyDashboardPage = () => {
   const uiConfig = useUiConfig();
+  const emitDataMasks = uiConfig?.emitDataMasks;
 
-  // Emit data mask changes to the parent window
-  if (uiConfig?.emitDataMasks) {
+  // Emit data mask changes to the parent window. Subscribing inside an effect
+  // (rather than during render) ensures the unsubscribe runs on unmount,
+  // including StrictMode's dev-mode double-mount cycle.
+  useEffect(() => {
+    if (!emitDataMasks) return undefined;
     log('setting up Switchboard event emitter');
 
     let previousDataMask = store.getState().dataMask;
 
-    store.subscribe(() => {
-      const currentState = store.getState();
-      const currentDataMask = currentState.dataMask;
-
-      // Only emit if the dataMask has changed
+    return store.subscribe(() => {
+      const currentDataMask = store.getState().dataMask;
       if (previousDataMask !== currentDataMask) {
         Switchboard.emit('observeDataMask', {
           ...currentDataMask,
@@ -88,7 +89,7 @@ const EmbededLazyDashboardPage = () => {
         previousDataMask = currentDataMask;
       }
     });
-  }
+  }, [emitDataMasks]);
 
   return <LazyDashboardPage idOrSlug={bootstrapData.embedded!.dashboard_id} />;
 };