feat(canvas): add visual annotation edit action

Add a VisualAnnotationPanel letting users mark regions on an image (brush,
circle, rectangle, cross) with per-mark edit instructions, composited into
an annotation guide image. The new "visual-annotation" node action sends
the unmarked source plus the annotated guide to the image model, editing
only numbered regions. Server validates the annotation image/instructions
and threads them through the generator task.

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

server/internal/domain/design/project.go

@@ -158,13 +158,14 @@ type Node struct {
 }
 
 type NodeActionRequest struct {
-	Action     string
-	Prompt     string
-	ImageModel string
-	ImageSize  string
-	Mask       string
-	Selection  string
-	Options    map[string]string
+	Action          string
+	Prompt          string
+	ImageModel      string
+	ImageSize       string
+	Mask            string
+	Selection       string
+	AnnotationImage string
+	Options         map[string]string
 }
 
 type Connection struct {