Restructure frontend code to allow for extensions (#4721)

# Description of Changes
Move frontend code into `core` folder and add infrastructure for
`proprietary` folder to include premium, non-OSS features

frontend/src/core/hooks/useFocusTrap.ts

@@ -0,0 +1,76 @@
+import { useEffect, RefObject } from 'react';
+
+const FOCUSABLE_ELEMENTS = [
+  'a[href]',
+  'button:not([disabled])',
+  'input:not([disabled])',
+  'select:not([disabled])',
+  'textarea:not([disabled])',
+  '[tabindex]:not([tabindex="-1"])',
+].join(', ');
+
+export function useFocusTrap(containerRef: RefObject<HTMLElement | null>, enabled: boolean = true) {
+  useEffect(() => {
+    if (!enabled || !containerRef.current) {
+      return;
+    }
+
+    const container = containerRef.current;
+    const getFocusableElements = () =>
+      Array.from(container.querySelectorAll<HTMLElement>(FOCUSABLE_ELEMENTS));
+
+    const handleKeyDown = (event: KeyboardEvent) => {
+      if (event.key !== 'Tab') {
+        return;
+      }
+
+      const focusableElements = getFocusableElements();
+      if (focusableElements.length === 0) {
+        return;
+      }
+
+      const firstElement = focusableElements[0];
+      const lastElement = focusableElements[focusableElements.length - 1];
+      const activeElement = document.activeElement as HTMLElement;
+
+      // Check if focus is within the container
+      if (!container.contains(activeElement)) {
+        event.preventDefault();
+        firstElement.focus();
+        return;
+      }
+
+      // Shift + Tab (backwards)
+      if (event.shiftKey) {
+        if (activeElement === firstElement) {
+          event.preventDefault();
+          lastElement.focus();
+        }
+      }
+      // Tab (forwards)
+      else {
+        if (activeElement === lastElement) {
+          event.preventDefault();
+          firstElement.focus();
+        }
+      }
+    };
+
+    // Focus first element on mount
+    const focusableElements = getFocusableElements();
+    if (focusableElements.length > 0) {
+      // Small delay to ensure the element is fully rendered
+      setTimeout(() => {
+        focusableElements[0]?.focus();
+      }, 100);
+    }
+
+    document.addEventListener('keydown', handleKeyDown);
+
+    return () => {
+      document.removeEventListener('keydown', handleKeyDown);
+    };
+  }, [containerRef, enabled]);
+}
+
+