fix(HEL-488): 盘中日K补上今天实时变化的一根

悬浮窗和详情页在 Tushare rt_k / iFinD 不可用时，改用免费实时行情或当日分时生成今日K，收盘后正式日K就绪再无缝替换。

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: multica-agent <github@multica.ai>

frontend/pages/market/preview.js

@@ -367,7 +367,11 @@ function selectStockPreviewChart(chart) {
     }
   } else if ((payload.prices || []).length) {
     setText("stockPreviewDate", payload.meta?.trade_date || "最新行情");
-    setText("stockPreviewSource", `日 K 行情 · ${payload.prices.length} 个交易日`);
+    const notice = String(payload.meta?.notice || "").trim();
+    setText(
+      "stockPreviewSource",
+      notice ? `日 K 行情 · ${payload.prices.length} 个交易日 · ${notice}` : `日 K 行情 · ${payload.prices.length} 个交易日`,
+    );
     drawDailyPreviewChart(payload.prices);
   } else {
     setText("stockPreviewDate", payload.meta?.trade_date || "最新行情");