fix(canvas): 画布背板 - 明确背板能力说明并允许拖动 Markdown 节点

背板能力卡改为说明可移动、可折叠的视觉分区用途与边界，Agent 能据此判断何时用背板归组。Markdown 节点正文不再拦截 mousedown，拖动节点和拖入背板恢复正常。

backend/internal/app/cloud_agent_storyboard_test.go

@@ -102,6 +102,16 @@ func TestCloudAgentNodeCapabilityCardsExplainStoryboardTradeoffs(t *testing.T) {
 	if !strings.Contains(videoGoodFor, "多图参考视频") {
 		t.Fatalf("video capability card does not explain multi-image video: %s", videoGoodFor)
 	}
+	frame := byType["frame"]
+	frameGoodFor := strings.Join(frame["goodFor"].([]string), "\n")
+	frameNotIdealFor := strings.Join(frame["notIdealFor"].([]string), "\n")
+	if !strings.Contains(frameGoodFor, "按场景或镜头组") || !strings.Contains(frameGoodFor, "制作") || !strings.Contains(frameNotIdealFor, "分镜脚本节点") {
+		t.Fatalf("frame capability card does not explain its use cases and limits: %+v", frame)
+	}
+	capabilityGuide := cloudAgentCapabilityGuide()
+	if !strings.Contains(capabilityGuide, "按场景或镜头组") || !strings.Contains(capabilityGuide, "可移动、可折叠") {
+		t.Fatalf("Agent system guide does not explain when to use a canvas frame: %s", capabilityGuide)
+	}
 	guide := strings.Join(result["selectionGuide"].([]string), "\n")
 	if !strings.Contains(guide, "多镜头") || !strings.Contains(guide, "model_list") || !strings.Contains(guide, "不要为了形式") {
 		t.Fatalf("selection guide does not express soft routing: %s", guide)