[feature] Add a download button for displayed DataFrame in Python block

As discussed in issue #18 (see detailed feature [here](https://github.com/briefercloud/briefer/issues/18#issuecomment-2346793149)), it would be great to have a button that allows users to download tables instantly as CSV files.

### Details

- Add a download button: place a download button next to each table in the UI.
- Instant CSV export: clicking the button should export the table data in CSV format immediately.
- Include Table State: the exported CSV should reflect the current state of the table, including transformations applied by the user (if they're available in the future).

Read the detailed feature discussion in issue #18, then locate the UI component that displays DataFrames in Python blocks and its table data handling. Add the per-table CSV download behavior described here, preserving the currently displayed table state, and verify that clicking the button produces the expected CSV.

Written by the indexing model from the issue text.