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/domain/design/agent.go

@@ -107,14 +107,16 @@ type GeneratorTaskQueueInfo struct {
 }
 
 type GeneratorTaskArtifactMetadata struct {
-	ArtifactID string
-	Format     string
-	Height     int64
-	Label      string
-	NodeID     string
-	NodeName   string
-	SizeBytes  int64
-	Width      int64
+	ArtifactID    string
+	BBox          []int64
+	Format        string
+	GeneratorName string
+	Height        int64
+	Label         string
+	NodeID        string
+	NodeName      string
+	SizeBytes     int64
+	Width         int64
 }
 
 type GeneratorTaskArtifact struct {