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。

web/src/components/canvas/nodes/markdown-node.tsx

@@ -39,7 +39,6 @@ export function MarkdownNodeContent({ node, theme }: MarkdownNodeContentProps) {
             data-canvas-no-zoom
             style={{ color: theme.node.text }}
             onWheel={(event) => event.stopPropagation()}
-            onMouseDown={(event) => event.stopPropagation()}
         >
             <AIMessageMarkdown>{source}</AIMessageMarkdown>
         </div>