feat: 新增表格文件预览，优化知识图谱与 Agent 详情交互

- 新增 spreadsheet-preview 组件及 xlsx 解析工具，支持产物中的表格文件预览
- 优化知识图谱画布、详情面板及样式
- 调整 Agent 详情面板、聊天与消息列表交互

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend-web/src/components/knowledge-graph/GraphDetailPanel.tsx

@@ -125,29 +125,22 @@ function DetailBody(props: GraphDetailPanelProps & { node: GraphNode }) {
             ? ` · ${node.category}`
             : ""}
         </span>
-        <span
-          className="kg-detail-type"
-          style={{ background: "rgba(148, 163, 184, 0.12)", color: "#cbd5e1" }}
-        >
+        <span className="kg-detail-type kg-detail-type-neutral">
           度数 {props.degree}
         </span>
         {props.explore && props.explore.depth !== null ? (
           <>
-            <span
-              className="kg-detail-type"
-              style={{ background: "rgba(56, 189, 248, 0.14)", color: "#7dd3fc" }}
-            >
+            <span className="kg-detail-type kg-detail-type-primary">
               {props.explore.isRoot
                 ? "探索起点"
                 : `第 ${props.explore.depth} 层`}
             </span>
             <span
-              className="kg-detail-type"
-              style={
+              className={`kg-detail-type ${
                 props.explore.expanded
-                  ? { background: "rgba(52, 211, 153, 0.14)", color: "#6ee7b7" }
-                  : { background: "rgba(251, 191, 36, 0.14)", color: "#fcd34d" }
-              }
+                  ? "kg-detail-type-success"
+                  : "kg-detail-type-warn"
+              }`}
             >
               {props.explore.expanded ? "已展开" : "可展开"}
             </span>