fix(web): normalize null trace arrays before render

ProjectTracePanel reads index.length; a cached or older payload of null must not crash the page.

Co-authored-by: Cursor <cursoragent@cursor.com>

web/src/api/projectTrace.ts

@@ -40,7 +40,13 @@ export async function fetchProjectTrace(projectId: number): Promise<ProjectTrace
   if (!res.ok) {
     throw new Error(`需求与影响加载失败: ${res.status}`);
   }
-  return res.json();
+  const data = await res.json();
+  return {
+    items: Array.isArray(data.items) ? data.items : [],
+    hidden_count: typeof data.hidden_count === 'number' ? data.hidden_count : 0,
+    index: Array.isArray(data.index) ? data.index : [],
+    recheck_marks: Array.isArray(data.recheck_marks) ? data.recheck_marks : [],
+  };
 }
 
 export async function clearRecheckMark(projectId: number, markId: number, reason: 'no_change' | 'revision_started'): Promise<void> {