fix(growth-quality): period select reads periods array from object response

Backend GET /growth-quality/periods returns {entity_id,periods[],coverage{}}; axios interceptor already unwraps response.data so the old r.data||r||[] put the whole object into el-select and Vue rendered JSON.

Also default to periods[0]: backend returns periods in DESC order, so old code picked the OLDEST period. Only frontend/src/views/GrowthQuality.vue changed.

frontend/src/views/GrowthQuality.vue

@@ -130,9 +130,13 @@ async function loadPeriods() {
   loadingPeriods.value = true
   try {
     const r: any = await api.get('/growth-quality/periods')
-    periods.value = r.data || r || []
+    // 后端返回 {entity_id, periods[], coverage{}}；axios 拦截器已 unwrap(response.data)，
+    // 故 r 即响应体本体、r.data 恒为 undefined（本页曾因此把整个对象塞进 el-select → 渲染成 JSON）
+    const raw = r?.periods ?? r?.data?.periods
+    const list = Array.isArray(raw) ? raw : (Array.isArray(r) ? r : [])
+    periods.value = list.filter((p: any) => typeof p === 'string')
     if (periods.value.length && !periods.value.includes(period.value)) {
-      period.value = periods.value[periods.value.length - 1]
+      period.value = periods.value[0]  // 后端按期间倒序返回 → 首元素即最新有数据的期间
     }
   } catch (_) {}
   loadingPeriods.value = false