feat(web): M9-4 Stock Research Page /stocks/[symbol]（v3 §20.4）

- 个股统一研究页：K 线（chart API，none/qfq/hfq 显示切换）+ 量 + MA +
  成交 fills/未成交信号标记 + 该股历史选股命中与信号记录（点击看原因/价格）
- 股票池每行代码可点击进入本页
- 数据全部来自 Chart API（前端不重算）；类型补齐 Stock.market/exchange
- 冒烟：/stocks/600519.SH 200，MySQL 上 qfq K 线返回正常；tsc --noEmit 通过

frontend/web/app/stocks/page.tsx

@@ -1,5 +1,6 @@
 "use client";
 
+import Link from "next/link";
 import { useCallback, useEffect, useRef, useState } from "react";
 import { apiGet } from "@/lib/api";
 import type { Stock } from "@/lib/types";
@@ -129,7 +130,7 @@ export default function StocksPage() {
             <tbody>
               {stocks.map((s) => (
                 <tr key={s.symbol}>
-                  <td className="cell-mono cell-strong">{s.symbol}</td>
+                  <td className="cell-mono cell-strong"><Link href={`/stocks/${s.symbol}`} style={{ textDecoration: "none" }}>{s.symbol}</Link></td>
                   <td>{s.name}</td>
                   <td>
                     <span className="tag tag--mono">{marketOf(s.symbol)}</span>