Load example datasets & workflows to dashboard

### Feature Summary

When users install Texera for the 1st time, the dashboard is empty with no example workflow or dataset to get started quickly.

We should have a mechanism to load the example workflows and datasets. I made few tryouts, e.g. #4075 But the loading is pretty ad-hoc, with no systematic design

### Proposed Solution or Design

Open to discuss

### Impact / Priority

(P2)Medium – useful enhancement

### Affected Area

_No response_

Start with the dashboard behavior described here and review the earlier trial in issue #4075. Define how first-time installations should load example workflows and datasets, including what “done” means for a systematic design rather than an ad-hoc mechanism.

Written by the indexing model from the issue text.