[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_

Start by tracing the frontend canvas-resize observer and the REMOTE-mode server-side render window, interactor, and orientation widget resize trigger. Verify the existing render-coalescing behavior for rapid resize events, then confirm that resizing fills the canvas without distortion and keeps the orientation widget proportional.

Written by the indexing model from the issue text.