fix(growth-quality): default period to newest period from API

The component shipped a hardcoded initial period (2026-06) that is present in the periods list, so the previous guard kept it and the page never defaulted to the newest period. Initialize empty so loadPeriods() applies periods[0] (backend returns DESC); /diagnosis treats an empty period as 'newest with data'.

frontend/src/views/GrowthQuality.vue

@@ -84,7 +84,7 @@ import { sessionEntityName } from '@/utils/entity'
 
 // 企业只读标识（账套由登录窗口选定，页内无切换入口）
 const entityLabel = ref(sessionEntityName())
-const period = ref('2026-06')
+const period = ref('')  // 空 = 由 loadPeriods() 取后端倒序首元素（最新有数据期间）
 const periods = ref<string[]>([])
 const loading = ref(false)
 const loadingPeriods = ref(false)