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>

frontend/src/domain/design.ts

@@ -268,6 +268,7 @@ export type DeleteProjectsResponse = {
 export type NodeActionRequest = {
   action:
     | "quick-edit"
+    | "visual-annotation"
     | "upscale"
     | "remove-background"
     | "eraser"
@@ -285,6 +286,7 @@ export type NodeActionRequest = {
   imageSize?: string;
   mask?: string;
   selection?: string;
+  annotationImage?: string;
   options?: Record<string, string>;
 };