feat(share): symbol page share button + distinguish note delete vs failure

- New ShareButton in the page header tries navigator.share() first
  (native sheet on mobile), then falls back to navigator.clipboard, then
  to a hidden textarea + execCommand("copy") for non-secure-context LAN.
  Shows a brief "링크 복사됨" / "복사 안됨" toast; native share success
  shows none (the OS sheet is the feedback).
- Mounted between ⇄ 비교 and the Star button.

Fixes reviewer concern on 442cc38: writeNote() used to return null for
both "user emptied the textarea (intentional delete)" and "localStorage
quota failure," so clearing a note showed "저장 안됨." It now returns a
discriminated { status: "saved" | "deleted" | "failed" } and the panel
shows "저장됨" / "삭제됨" / "저장 안됨" accordingly. Failure keeps the
user's text so they can retry or shorten it.

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

web/components/InvestmentNote.tsx

@@ -7,6 +7,12 @@
 import { useEffect, useMemo, useState } from "react";
 import { NOTE_MAX_LEN, readNote, writeNote, type Note } from "../lib/notes";
 
+const FEEDBACK_TEXT: Record<"saved" | "deleted" | "failed", string> = {
+  saved: "저장됨",
+  deleted: "삭제됨",
+  failed: "저장 안됨",
+};
+
 type Props = {
   code: string;
 };
@@ -60,10 +66,16 @@ export function InvestmentNote({ code }: Props) {
   const remaining = useMemo(() => NOTE_MAX_LEN - text.length, [text]);
 
   const save = () => {
-    const next = writeNote(code, text);
-    setSaved(next);
-    setText(next?.text ?? "");
-    setFeedback(next ? "저장됨" : "저장 안됨");
+    const res = writeNote(code, text);
+    if (res.status === "saved") {
+      setSaved(res.note);
+      setText(res.note.text);
+    } else if (res.status === "deleted") {
+      setSaved(null);
+      setText("");
+    }
+    // failed: 입력값 유지 — 사용자가 재시도하거나 일부를 줄일 수 있도록.
+    setFeedback(FEEDBACK_TEXT[res.status]);
   };
 
   return (

web/components/ShareButton.tsx

@@ -0,0 +1,100 @@
+"use client";
+
+// 종목 페이지 공유 버튼 — 현재 URL 을 클립보드에 복사.
+// Toss 의 종목 공유 패턴 (개별 모달 없이 즉시 복사 + 짧은 토스트).
+//
+// 폴백 순서:
+//   1) navigator.clipboard.writeText (HTTPS or localhost)
+//   2) document.execCommand("copy") (legacy, http LAN 등 secure context 아닌 경우)
+//   3) 두 경로 모두 실패면 "복사 안됨" 안내
+
+import { useEffect, useState } from "react";
+
+type Props = {
+  code: string;
+  name?: string;
+};
+
+async function copyToClipboard(text: string): Promise<boolean> {
+  // Modern path — secure context 에서만 사용 가능.
+  if (typeof navigator !== "undefined" && navigator.clipboard?.writeText) {
+    try {
+      await navigator.clipboard.writeText(text);
+      return true;
+    } catch {
+      // 권한 거부 등 — fallback 시도.
+    }
+  }
+  // Legacy fallback. document.execCommand 는 deprecated 지만 비-HTTPS LAN 환경 호환용.
+  if (typeof document === "undefined") return false;
+  try {
+    const ta = document.createElement("textarea");
+    ta.value = text;
+    ta.setAttribute("readonly", "");
+    ta.style.position = "fixed";
+    ta.style.opacity = "0";
+    document.body.appendChild(ta);
+    ta.select();
+    const ok = document.execCommand("copy");
+    document.body.removeChild(ta);
+    return ok;
+  } catch {
+    return false;
+  }
+}
+
+export function ShareButton({ code, name }: Props) {
+  const [feedback, setFeedback] = useState<string | null>(null);
+
+  useEffect(() => {
+    if (!feedback) return;
+    const h = window.setTimeout(() => setFeedback(null), 2000);
+    return () => window.clearTimeout(h);
+  }, [feedback]);
+
+  const onClick = async () => {
+    if (typeof window === "undefined") return;
+    const url = window.location.href;
+    // Web Share API 우선 — 모바일에서 native sheet 뜸. 데스크탑/실패 시 클립보드로 폴백.
+    const shareData = {
+      title: name ? `${name} (${code})` : code,
+      text: name ? `${name} 종목 정보` : code,
+      url,
+    };
+    if (
+      typeof navigator !== "undefined" &&
+      typeof navigator.share === "function"
+    ) {
+      try {
+        await navigator.share(shareData);
+        return; // 사용자가 native sheet 에서 처리. 토스트 안 띄움.
+      } catch {
+        // AbortError(사용자 취소) 또는 미지원 — 클립보드로 폴백.
+      }
+    }
+    const ok = await copyToClipboard(url);
+    setFeedback(ok ? "링크 복사됨" : "복사 안됨");
+  };
+
+  return (
+    <div className="relative">
+      <button
+        type="button"
+        onClick={onClick}
+        className="rounded-md border border-zinc-700 px-2 py-0.5 text-[11px] text-zinc-400 transition hover:border-zinc-500 hover:text-zinc-200"
+        title="이 종목 페이지 공유"
+        aria-label="이 종목 페이지 공유"
+      >
+        ↗ 공유
+      </button>
+      {feedback && (
+        <span
+          aria-live="polite"
+          className="pointer-events-none absolute right-0 top-full z-20 mt-1 whitespace-nowrap rounded-md border border-zinc-700 bg-zinc-950/90 px-2 py-0.5 text-[10px] text-zinc-200 shadow"
+        >
+          {feedback}
+        </span>
+      )}
+    </div>
+  );
+}