feat: 全部页面添加 MonthPicker 月份切换

新增 MonthPicker 的页面(6个):
- CostPage: 成本与库存管理
- SKUPage: 商品SKU管理
- PlatformPage: 平台与优惠管理
- MemberPage: 会员与复购管理
- SiteSelectionPage: 门店选址分析
- DataQualityPage: 数据质量看板

容器+子组件添加 MonthPicker(2个页面, 11个子组件):
- StoreExpensePage: 门店营业费用分析(10个子组件全部传month)
- CostAnalysisPage/StoreTab: 门店成本tab

跳过(后端不支持month参数):
- RiskPage: 风险管理(全量视图, 不按月过滤)
- TimePage: 时间段分析(全量视图, 不按月过滤)

client/src/components/store-expense/FixedVariableTab.tsx

@@ -9,12 +9,12 @@ import { formatCurrency, formatPercent } from '@/lib/utils'
 
 const PAGE_SIZE = 20
 
-export function FixedVariableTab() {
+export function FixedVariableTab({ month }: { month: string }) {
   const [page, setPage] = useState(1)
   const [sort, setSort] = useState('fixed_rate_pct')
   const [filter, setFilter] = useState('')
   const [order, setOrder] = useState('desc')
-  const { data, isLoading } = useQuery({ queryKey: ['se/fixed-variable', page, sort, filter, order], queryFn: () => api.get(`/store-expense/fixed-variable?page=${page}&page_size=${PAGE_SIZE}&sort=${sort}&filter=${filter}&order=${order}`) })
+  const { data, isLoading } = useQuery({ queryKey: ['se/fixed-variable', month, page, sort, filter, order], queryFn: () => api.get(`/store-expense/fixed-variable?page=${page}&page_size=${PAGE_SIZE}&sort=${sort}&filter=${filter}&order=${order}&month=${month}`) })
 
   if (isLoading) return <LoadingSpinner text="加载固定/变动费用拆分..." />