feat: add Grid3 image support for web viewer

- Store gridPageName in button parameters for Grid3 files
- Store position (x, y) on buttons (already existed)
- Enables web viewers to fetch Grid3 images via API endpoint

Author: willwade

src/processors/gridsetProcessor.ts

@@ -1399,6 +1399,8 @@ class GridsetProcessor extends BaseProcessor {
                   pluginMetadata.autoContentType === 'Prediction'
                     ? predictionCellCounter
                     : undefined,
+                // Store page name for Grid3 image lookup
+                gridPageName: gridName,
               },
             });