refactor context menu and keyboard navigation handling for improved clarity and maintainability

dicom-viewer/src/App.tsx

@@ -1926,6 +1926,8 @@ useEffect(() => {
 
 
   useEffect(() => {
+    const el = appRootRef.current;
+    if (!el) return;
     function handleArrowKeyNavigation(e: KeyboardEvent) {
       console.log("Handling arrow key navigation", e.key);
       console.log("activeViewport", activeViewport);
@@ -1975,8 +1977,11 @@ useEffect(() => {
       }
     }
 
-    window.addEventListener("keydown", handleArrowKeyNavigation);
-    return () => window.removeEventListener("keydown", handleArrowKeyNavigation);
+    el.addEventListener("keydown", handleArrowKeyNavigation);
+
+    return () => {
+      el.removeEventListener("keydown", handleArrowKeyNavigation);
+    };
   }, [activeViewport, viewportModes, viewportImageIds, updateOverlay]);