fix(admin-web): handle undefined data in dashboard hooks

Add null-safe access and fallback to empty arrays to prevent
"Cannot read properties of undefined" errors when API returns
unexpected data structure.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/admin-web/src/hooks/useDashboard.ts

@@ -40,7 +40,8 @@ export function useDashboardStats() {
     queryKey: dashboardKeys.stats(),
     queryFn: async () => {
       const response = await dashboardService.getStats();
-      return response.data.stats;
+      // 确保返回数组，即使 API 返回 undefined 或非数组
+      return response?.data?.stats ?? [];
     },
     staleTime: 30 * 1000,
     gcTime: 5 * 60 * 1000,
@@ -71,7 +72,8 @@ export function useDashboardRegion() {
     queryKey: dashboardKeys.region(),
     queryFn: async () => {
       const response = await dashboardService.getRegionDistribution();
-      return response.data.regions;
+      // 确保返回数组
+      return response?.data?.regions ?? [];
     },
     staleTime: 5 * 60 * 1000, // 5分钟后标记为过期
     gcTime: 10 * 60 * 1000,
@@ -87,7 +89,8 @@ export function useDashboardActivities(limit = 5) {
     queryKey: dashboardKeys.activities(limit),
     queryFn: async () => {
       const response = await dashboardService.getRecentActivities(limit);
-      return response.data.activities;
+      // 确保返回数组
+      return response?.data?.activities ?? [];
     },
     staleTime: 30 * 1000, // 30秒后标记为过期
     gcTime: 5 * 60 * 1000,