feat: Refactor Google Drive and Google Photos authentication handling

- Separate built-in authentication options for source and destination in configuration forms.
- Update UI components to reflect changes in authentication handling for Google Drive and Google Photos.
- Implement path validation for local directories in the configuration forms, enhancing user experience.
- Revise README to clarify supported source and destination types, including updated terminology for Google Drive.
- Introduce new API endpoint for path validation, improving error handling and user feedback.

README.md

@@ -348,8 +348,8 @@ User management features:
 ### Transfer Configuration Options
 
 1. **Source/Destination Types**:
-   - Google Drive (with built-in or custom authentication)
-   - Google Photos (with built-in or custom authentication)
+   - Google Drive
+   - Google Photos
    - Local filesystem
    - Amazon S3
    - MinIO (S3-compatible storage)