[Remote rendering 4.6] Viewport resize in REMOTE mode

### 📝 Description of the feature

Keep the server-side render window in step with the canvas size so the stream isn't stretched and the orientation widget stays proportional. A resize trigger updates the render window, interactor, and orientation widge; the frontend observes canvas resize and fires it.

Note: rapid resize events during a drag shouldn't trigger a render per event. Whether that's handled by the existing render coalescing or its own rate-limiting is a call for this story; not a settled requirement.

### Acceptance Criteria

After a resize the stream fills the canvas without distortion and the orientation widget stays proportional.

### 💵 Business Value

_No response_

### 🔗 Useful links and references

_No response_

首先追蹤前端 canvas-resize observer，以及 REMOTE 模式下伺服器端的 render window、interactor 和 orientation widget resize trigger。驗證快速 resize 事件的現有 render-coalescing 行為，然後確認 resize 能在不產生失真的情況下填滿 canvas，並維持 orientation widget 的比例。

由索引模型根據 Issue 內容生成。