概念/行业分析添加日期选择功能

- backend: market_snapshot 支持可选 as_of 参数
- frontend: 两页加 DatePicker，按选定日期查询数据

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

serve/frontend/src/lib/api.ts

@@ -882,8 +882,8 @@ export const api = {
         ? `/api/screener/cached?ext_columns=${encodeURIComponent(extColumns)}`
         : '/api/screener/cached',
     ),
-  marketSnapshot: () =>
-    request<{ as_of: string | null; rows: MarketSnapshotRow[] }>('/api/screener/market-snapshot'),
+  marketSnapshot: (asOf?: string) =>
+    request<{ as_of: string | null; rows: MarketSnapshotRow[] }>(`/api/screener/market-snapshot${asOf ? `?as_of=${asOf}` : ''}`),
   overviewMarket: (asOf?: string) => request<OverviewMarket>(`/api/overview/market${asOf ? `?as_of=${asOf}` : ''}`),
 
   // 概念涨幅轮动矩阵: 每列(日期)各自把所有概念按当天涨幅从高到低排序