feat(server): add layer-separation generator task

Introduce the `layer-separation` generator (edit-elements node action)
that splits a source image into a clean background layer, foreground
image artifacts with bounding boxes, and editable text render data.

- New layer_separation.go builds the separated result and artifacts
- SubmitGeneratorTask routes layer-separation and move-object via a
  switch; standalone task completion + polling support
- Extend artifact metadata with BBox and GeneratorName across the
  domain, API types, mapper, .api schema, and API.md
- edit-elements gets a transparent manual-frame placeholder holding the
  separated foreground images and text layers beside the original

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/logic/mapper.go

@@ -133,14 +133,16 @@ func toAPIGeneratorTaskArtifacts(items []design.GeneratorTaskArtifact) []types.G
 			Type:    item.Type,
 			Content: item.Content,
 			Metadata: types.GeneratorTaskArtifactMetadata{
-				ArtifactId: item.Metadata.ArtifactID,
-				Format:     item.Metadata.Format,
-				Height:     item.Metadata.Height,
-				Label:      item.Metadata.Label,
-				NodeId:     item.Metadata.NodeID,
-				NodeName:   item.Metadata.NodeName,
-				SizeBytes:  item.Metadata.SizeBytes,
-				Width:      item.Metadata.Width,
+				ArtifactId:    item.Metadata.ArtifactID,
+				BBox:          append([]int64(nil), item.Metadata.BBox...),
+				Format:        item.Metadata.Format,
+				GeneratorName: item.Metadata.GeneratorName,
+				Height:        item.Metadata.Height,
+				Label:         item.Metadata.Label,
+				NodeId:        item.Metadata.NodeID,
+				NodeName:      item.Metadata.NodeName,
+				SizeBytes:     item.Metadata.SizeBytes,
+				Width:         item.Metadata.Width,
 			},
 		})
 	}