[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_

先從伺服器端操作 add_dataset、update_variables 和 load_state 開始，然後追蹤用於場景刷新的遠端模式用戶端路徑。驗證透過 Python API 進行的場景變更是否會在不重新載入瀏覽器的情況下更新算繪的畫面以及相關的樹狀結構、圖例、色彩面板或 widget UI，並執行專案測試。

由索引模型根據 Issue 內容生成。