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/backend/app/api/screener.py

@@ -296,13 +296,14 @@ def get_cached(
 
 
 @router.get("/market-snapshot")
-def market_snapshot(request: Request):
-    """最新全市场轻量行情快照，供板块/概念聚合分析使用。"""
+def market_snapshot(request: Request, as_of: date | None = None):
+    """全市场轻量行情快照（可选指定日期），供板块/概念聚合分析使用。"""
     import polars as pl
 
     repo = request.app.state.repo
     svc = ScreenerService(repo)
-    as_of = svc.latest_date()
+    if as_of is None:
+        as_of = svc.latest_date()
     if not as_of:
         return {"as_of": None, "rows": []}