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/img_infinite_canvas.api

@@ -781,15 +781,16 @@ type GenerateResponse {
 }
 
 type NodeActionRequest {
-	Id         string            `path:"id"`
-	NodeId     string            `path:"nodeId"`
-	Action     string            `json:"action"`
-	Prompt     string            `json:"prompt,optional"`
-	ImageModel string            `json:"imageModel,optional"`
-	ImageSize  string            `json:"imageSize,optional"`
-	Mask       string            `json:"mask,optional"`
-	Selection  string            `json:"selection,optional"`
-	Options    map[string]string `json:"options,optional"`
+	Id              string            `path:"id"`
+	NodeId          string            `path:"nodeId"`
+	Action          string            `json:"action"`
+	Prompt          string            `json:"prompt,optional"`
+	ImageModel      string            `json:"imageModel,optional"`
+	ImageSize       string            `json:"imageSize,optional"`
+	Mask            string            `json:"mask,optional"`
+	Selection       string            `json:"selection,optional"`
+	AnnotationImage string            `json:"annotationImage,optional"`
+	Options         map[string]string `json:"options,optional"`
 }
 
 type PrepareMockupSurfaceRequest {