Result download button does nothing when export is disabled

### What happened?

On a default local deployment (bin/local-dev.sh or bin/single-node), results cannot be downloaded, and nothing tells the user why.

The download icon on every result cell renders regardless of configuration. Clicking it opens the export dialog, but none of its actions work: exporting to local, saving to an existing dataset, or saving to a newly created dataset. The dialog closes and nothing happens: no request is sent, no message is shown, and a newly created dataset stays empty. The cause is `export-execution-result-enabled`, which defaults to `false` in `common/config/src/main/resources/gui.conf`. `WorkflowResultExportService.performExport` returns early on that flag without any notification, and the dialog closes regardless.

The rest of the UI already honours the switch: the top-menu Export entry is disabled and the context-menu entry is hidden. Only the per-cell download button was left out, so it looks like a broken feature rather than a disabled one. The k8s chart turns the switch on explicitly; single-node and local-dev do not, so those deployments lose the feature silently.

Expected: either downloading works out of the box, or the UI says clearly that result export is disabled on this deployment.

Proposal:
1. Make the per-cell download button honour the switch the same way the menu does (disabled, with a tooltip saying why). I have a small frontend fix with tests ready and can open a PR.
2. Consider defaulting the switch to true, since every interactive deployment already turns it on.

### How to reproduce?

1. Start Texera with the default gui.conf (bin/local-dev.sh up, or bin/single-node), so export-execution-result-enabled is false.
2. Run any workflow and open the result panel.
3. Click the cloud-download icon on any cell. The export dialog opens.
4. Try each destination: Export to local; select an existing dataset and save; create a new dataset and save.
5. The dialog closes and nothing happens. DevTools > Network shows no request. Meanwhile the top-menu Export entry is greyed out, which is the behaviour the cell button should share.

https://github.com/user-attachments/assets/c770a437-7547-4809-a1a6-88cad1013855

Setting GUI_WORKFLOW_WORKSPACE_EXPORT_EXECUTION_RESULT_ENABLED=true and restarting makes the same button work, which confirms the switch is the only cause.

### Version/Branch

1.4.0-incubating-SNAPSHOT (main)

### Commit Hash (Optional)

dcb25efa9e1d4321b7b99459db4ee777f37cbaff

### What browsers are you seeing the problem on?

_No response_

### Relevant log output

```shell
No request is sent; the export service returns before any HTTP call.
```