[Remote rendering 4.4] Live scene refresh upon server-originated operations in remote mode

### 📝 Description of the feature

Fullstack user story. Keep the client view in step with server-side scene changes so adding or removing a dataset on a live remote session refreshes the view without a browser reload.

When a scene-mutating operation originates on the server (ie through an API, eg add_dataset, update_variables, load_state), the client has no way to know unless the server tells it. This user story propagates those changes: the server signals that the scene changed, and the frontend re-fetches scene details and rebuilds so the view reflects the new structure.

Two main surfaces update:

- The rendered frame (the server re-renders it and the stream carries it), and the UI computed from scene structure, tree view, legend/color panel, and widgets, which the frame alone doesn't touch.
- The surfaces differ by operation, most involve a tree rebuild, while update_variables changes variable metadata and so the legend rather than the tree.

Fullstack: the backend signal and frontend rebuild are individually untestable, so these are together in a single user story here.

### Acceptance Criteria

A Python API call that modifies the scene, e.g. add_dataset, refreshes without a browser reload, and the corresponding UI updates (e.g. tree rebuild) works in remote mode.
Tests pass

### 💵 Business Value

_No response_

### 🔗 Useful links and references

_No response_

Start with the server-side operations add_dataset, update_variables, and load_state, then trace the remote-mode client path for scene refreshes. Verify that a Python API scene change updates the rendered frame and the relevant tree, legend, color panel, or widget UI without a browser reload, and run the project tests.

Rédigé par le modèle d'indexation à partir du texte de l'issue.