feat(chart): CSV export of current chart range + fix ShareButton comment

- New lib/csv.ts: RFC 4180 escaping, UTF-8 BOM prefix (한글 Excel 호환),
  Blob + temporary anchor download, filename
  `<name>_<code>_<interval>_<from>_<to>.csv` with filesystem-illegal
  characters replaced by `_`.
- StockChart toolbar now always renders (was hidden in 10m intraday mode
  because it gated on showSma) and includes a right-aligned `⬇ CSV`
  button next to the SMA/RSI/MACD chips. Button is disabled when ohlcv
  is empty.

Also addresses the reviewer's non-blocking note on f18493f:
ShareButton.tsx header comment claimed clipboard-first, but the code
calls navigator.share() first. Comment now matches the actual order:
share → clipboard → execCommand → "복사 안됨".

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/lib/csv.ts

@@ -0,0 +1,69 @@
+// OHLCV → CSV 변환 + 다운로드 트리거.
+// 차트 데이터를 분석가 친화 CSV (UTF-8 BOM, Excel 호환) 로 내려받게 해줌.
+
+import type { ChartInterval, OhlcvPoint } from "./api";
+
+function escapeField(v: string): string {
+  // 표준 RFC 4180 — 쉼표/큰따옴표/개행 포함 시 큰따옴표로 감싸고 내부 큰따옴표는 이중화.
+  if (v.includes(",") || v.includes('"') || v.includes("\n")) {
+    return `"${v.replace(/"/g, '""')}"`;
+  }
+  return v;
+}
+
+function num(n: number | null | undefined): string {
+  if (n == null || !Number.isFinite(n)) return "";
+  return String(n);
+}
+
+export function ohlcvToCsv(ohlcv: OhlcvPoint[]): string {
+  const header = ["date", "open", "high", "low", "close", "volume"];
+  const lines: string[] = [header.join(",")];
+  for (const p of ohlcv) {
+    lines.push(
+      [
+        escapeField(p.date),
+        num(p.open),
+        num(p.high),
+        num(p.low),
+        num(p.close),
+        num(p.volume),
+      ].join(","),
+    );
+  }
+  return lines.join("\n");
+}
+
+export function downloadCsv(filename: string, csv: string): boolean {
+  if (typeof window === "undefined" || typeof document === "undefined") return false;
+  // Excel BOM — 한글 Excel 이 UTF-8 로 추론하도록.
+  const BOM = "\uFEFF";
+  const blob = new Blob([BOM + csv], { type: "text/csv;charset=utf-8" });
+  const url = URL.createObjectURL(blob);
+  try {
+    const a = document.createElement("a");
+    a.href = url;
+    a.download = filename;
+    a.style.display = "none";
+    document.body.appendChild(a);
+    a.click();
+    document.body.removeChild(a);
+    return true;
+  } catch {
+    return false;
+  } finally {
+    setTimeout(() => URL.revokeObjectURL(url), 0);
+  }
+}
+
+// `삼성전자_005930_1d_2025-08-01_2025-11-28.csv` — 파일시스템 금지 문자만 `_` 치환.
+export function csvFilename(
+  name: string,
+  code: string,
+  interval: ChartInterval,
+  from: string,
+  to: string,
+): string {
+  const safe = (s: string) => s.replace(/[\\/:*?"<>|]/g, "_").trim() || "data";
+  return `${safe(name)}_${code}_${interval}_${from}_${to}.csv`;
+}