Add image extraction functionality with background removal

- Implemented ExtractImageRegion operator for extracting image regions using box or lasso selection.
- Added RemoveImageRegionBackground job for background removal from selected image regions.
- Created utility functions for polygon bounds, selection canvas creation, and applying selection masks.
- Integrated image processing using PIL and numpy for efficient pixel manipulation.
- Enhanced user experience with modal drawing and status updates during extraction.

docs/stylesheets/extra.css

@@ -11,3 +11,17 @@
 .md-typeset .grid.cards > ul > li {
   border-radius: 0.5rem;
 }
+
+.md-typeset .screenshot-placeholder {
+  margin: 1rem 0;
+  padding: 2.5rem 1.25rem;
+  border: 2px dashed var(--md-default-fg-color--lighter);
+  border-radius: 0.5rem;
+  color: var(--md-default-fg-color--light);
+  text-align: center;
+  background: var(--md-code-bg-color);
+}
+
+.md-typeset .screenshot-placeholder p {
+  margin: 0.35rem 0;
+}