fix(frontend): prevent memory leak from uncleaned storage listener in useFeatureFlag

[Mercy017]

Root cause

useFeatureFlag called window.addEventListener('storage', evaluate) inside useIsomorphicLayoutEffect but never returned a cleanup function. Each time flag or scrollTargetId changed, React re-ran the effect — adding a new listener without removing the previous one. After n re-renders, n orphaned listener references accumulated in window._eventListeners, each retaining the evaluate closure (and with it flag, scrollTargetId, setIsEnabled, etc.) in memory.

Fix (src/hooks/useFeatureFlag.ts)

1. Extract the body as a named evaluate() function so the same reference is passed to both addEventListener and removeEventListener (required for correct deregistration).
2. Return () => window.removeEventListener('storage', evaluate) from the effect so React cleans up on unmount and before every re-run triggered by a dep change.

-   const newEnabled = getFeatureFlag(flag);
-   setIsEnabled(newEnabled);
-   …
+   function evaluate() {
+     const newEnabled = getFeatureFlag(flag);
+     setIsEnabled(newEnabled);
+     …
+   }
+
+   evaluate();
+
+   if (typeof window !== 'undefined') {
+     window.addEventListener('storage', evaluate);
+     return () => window.removeEventListener('storage', evaluate);
+   }
  }, [flag, scrollTargetId]);

The storage listener also makes flag state reactive: toggling a flag via devtools or another tab now updates the component without a page reload.

Regression tests (src/hooks/__tests__/useFeatureFlag.test.ts)

• removes storage listener on unmount so it does not leak — spies on window.addEventListener/removeEventListener and asserts the exact handler registered is also removed when the hook unmounts.
• re-evaluates the flag when a storage event fires — dispatches a StorageEvent after changing the mock return value and asserts the hook state updates.

Closes #1220