feat(ui): Introduce useUI Hook and UIContext (#5488)

Co-authored-by: Jacob Richman <jacob314@gmail.com>

packages/cli/src/ui/contexts/AppContext.tsx

@@ -0,0 +1,22 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { createContext, useContext } from 'react';
+
+export interface AppState {
+  version: string;
+  startupWarnings: string[];
+}
+
+export const AppContext = createContext<AppState | null>(null);
+
+export const useAppContext = () => {
+  const context = useContext(AppContext);
+  if (!context) {
+    throw new Error('useAppContext must be used within an AppProvider');
+  }
+  return context;
+};