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/useGoogleDrivePicker.ts

@@ -0,0 +1,98 @@
+/**
+ * React hook for Google Drive file picker
+ */
+
+import { useState, useCallback, useEffect } from 'react';
+import {
+  getGoogleDrivePickerService,
+  isGoogleDriveConfigured,
+  getGoogleDriveConfig,
+} from '@app/services/googleDrivePickerService';
+
+interface UseGoogleDrivePickerOptions {
+  multiple?: boolean;
+  mimeTypes?: string;
+}
+
+interface UseGoogleDrivePickerReturn {
+  isEnabled: boolean;
+  isLoading: boolean;
+  error: string | null;
+  openPicker: (options?: UseGoogleDrivePickerOptions) => Promise<File[]>;
+}
+
+/**
+ * Hook to use Google Drive file picker
+ */
+export function useGoogleDrivePicker(): UseGoogleDrivePickerReturn {
+  const [isEnabled, setIsEnabled] = useState(false);
+  const [isLoading, setIsLoading] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+  const [isInitialized, setIsInitialized] = useState(false);
+
+  // Check if Google Drive is configured on mount
+  useEffect(() => {
+    const configured = isGoogleDriveConfigured();
+    setIsEnabled(configured);
+  }, []);
+
+  /**
+   * Initialize the Google Drive service (lazy initialization)
+   */
+  const initializeService = useCallback(async () => {
+    if (isInitialized) return;
+
+    const config = getGoogleDriveConfig();
+    if (!config) {
+      throw new Error('Google Drive is not configured');
+    }
+
+    const service = getGoogleDrivePickerService();
+    await service.initialize(config);
+    setIsInitialized(true);
+  }, [isInitialized]);
+
+  /**
+   * Open the Google Drive picker
+   */
+  const openPicker = useCallback(
+    async (options: UseGoogleDrivePickerOptions = {}): Promise<File[]> => {
+      if (!isEnabled) {
+        setError('Google Drive is not configured');
+        return [];
+      }
+
+      try {
+        setIsLoading(true);
+        setError(null);
+
+        // Initialize service if needed
+        await initializeService();
+
+        // Open picker
+        const service = getGoogleDrivePickerService();
+        const files = await service.openPicker({
+          multiple: options.multiple ?? true,
+          mimeTypes: options.mimeTypes,
+        });
+
+        return files;
+      } catch (err) {
+        const errorMessage = err instanceof Error ? err.message : 'Failed to open Google Drive picker';
+        setError(errorMessage);
+        console.error('Google Drive picker error:', err);
+        return [];
+      } finally {
+        setIsLoading(false);
+      }
+    },
+    [isEnabled, initializeService]
+  );
+
+  return {
+    isEnabled,
+    isLoading,
+    error,
+    openPicker,
+  };
+}