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/application/generator_task.go

@@ -786,6 +786,9 @@ func generatorTaskInputArgsForNodeAction(target design.Node, req design.NodeActi
 		Prompt:       generatorTaskPromptForNodeAction(req),
 		Resolution:   resolutionLabel(imageSize),
 	}
+	if normalizeNodeAction(req.Action) == "visual-annotation" && strings.TrimSpace(req.AnnotationImage) != "" {
+		input.Image = append(input.Image, strings.TrimSpace(req.AnnotationImage))
+	}
 	if normalizeNodeAction(req.Action) == "move-object" {
 		input.SrcBox, input.DstBox = moveObjectBoxesFromSelection(req.Selection)
 	}