Add Download and "Upload to server" buttons to the ROI panel. The HTTP
reader gains GetROIDefinitions (GET /config/roi) and UploadROIDefinitions
(PUT /config/roi), converting between ROIDefinition and the generated
Roi_definitions model (including the optional azimuthal phi sector), the
same shapes OpenAPIConvert uses on the server. Download applies the
fetched ROIs through SetROIDefinition; upload pushes the current ones.
Both are no-ops unless the viewer is connected to a broker.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>