Add Google Drive authorization processing and error handling templates

- Implemented `google_drive_callback.html` for processing Google Drive authorization, including UI for success and error states.
- Added JavaScript functionality for exchanging authorization codes, saving settings, and handling folder selection.
- Created `google_drive_callback_error.html` to display error messages during the authorization process.

frontend/templates/privacy.html

@@ -58,6 +58,9 @@
     <p class="text-gray-700">
       <strong>No Additional Sharing:</strong> We do not share, sell, or otherwise disclose your personal data to third parties for advertising or marketing purposes.
     </p>
+    <p class="text-gray-700 mt-3">
+      <strong>Cloud Storage Integration:</strong> We integrate with Google Drive, Dropbox, OneDrive and other storage providers to enable you to save and access your documents across services.
+    </p>
   </div>
 
   <div class="bg-white shadow rounded-lg p-6 mb-6">