feat(notes): per-code private investment note panel

- New lib/notes.ts: localStorage key `notes:<code>` storing { text, updatedAt },
  2000-char cap, blank/whitespace removes the key.
- New InvestmentNote sidebar panel: textarea + character counter, save button
  enabled only when dirty, "방금 전 / N분 전 / 오늘 HH:MM / YYYY-MM-DD"
  relative timestamp on the last save, short save-confirmation toast.
- Mounted in [code]/page.tsx sidebar after AlertsPanel.

Also tightens writeTargets() to require strictly positive finite values,
matching the UI's parseNum() guard so the lib is safe under direct external
calls (reviewer's non-blocking note on a4a4a7f).

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

web/app/[code]/page.tsx

@@ -5,6 +5,7 @@ import { useEffect, useMemo, useState } from "react";
 import { AlertsPanel } from "../../components/AlertsPanel";
 import { CompositeScoreCard } from "../../components/CompositeScoreCard";
 import { DisclosuresPanel } from "../../components/DisclosuresPanel";
+import { InvestmentNote } from "../../components/InvestmentNote";
 import { InvestorCumulative } from "../../components/InvestorCumulative";
 import { MetricsPanel } from "../../components/MetricsPanel";
 import { PeerComparePanel } from "../../components/PeerComparePanel";
@@ -198,6 +199,7 @@ export default function CodePage({ params }: { params: { code: string } }) {
               <OrderbookPanel code={code} />
               <PriceTargets code={code} current={current} onChange={setTargets} />
               <AlertsPanel code={code} name={chart?.name} current={current} />
+              <InvestmentNote code={code} />
               <RelatedStocks code={code} />
             </div>
           </div>

web/components/InvestmentNote.tsx

@@ -0,0 +1,107 @@
+"use client";
+
+// 종목 페이지의 사이드 메모 패널.
+// Toss "투자 메모" 패턴 — 진입 이유, 관전 포인트를 짧게 적는 자리.
+// localStorage 만 사용, 서버 전송 없음.
+
+import { useEffect, useMemo, useState } from "react";
+import { NOTE_MAX_LEN, readNote, writeNote, type Note } from "../lib/notes";
+
+type Props = {
+  code: string;
+};
+
+// '방금 전', '5분 전', '오늘 14:30', 'YYYY-MM-DD' 단계로 압축.
+function fmtUpdatedAt(iso: string): string {
+  const t = Date.parse(iso);
+  if (!Number.isFinite(t)) return "";
+  const now = Date.now();
+  const diffSec = Math.max(0, Math.floor((now - t) / 1000));
+  if (diffSec < 60) return "방금 전";
+  const diffMin = Math.floor(diffSec / 60);
+  if (diffMin < 60) return `${diffMin}분 전`;
+  const d = new Date(t);
+  const pad = (n: number) => String(n).padStart(2, "0");
+  const ymd = `${d.getFullYear()}-${pad(d.getMonth() + 1)}-${pad(d.getDate())}`;
+  const hm = `${pad(d.getHours())}:${pad(d.getMinutes())}`;
+  // 같은 날짜면 '오늘 HH:MM', 아니면 'YYYY-MM-DD'
+  const today = new Date();
+  if (
+    today.getFullYear() === d.getFullYear() &&
+    today.getMonth() === d.getMonth() &&
+    today.getDate() === d.getDate()
+  ) {
+    return `오늘 ${hm}`;
+  }
+  return ymd;
+}
+
+export function InvestmentNote({ code }: Props) {
+  const [text, setText] = useState("");
+  const [saved, setSaved] = useState<Note | null>(null);
+  const [feedback, setFeedback] = useState<string | null>(null);
+
+  useEffect(() => {
+    const n = readNote(code);
+    setSaved(n);
+    setText(n?.text ?? "");
+    setFeedback(null);
+  }, [code]);
+
+  const dirty = (saved?.text ?? "") !== text;
+
+  // 저장 직후 잠깐 띄우는 toast 같은 안내. dirty 또는 종목 전환 시 자동 해제.
+  useEffect(() => {
+    if (!feedback) return;
+    const h = window.setTimeout(() => setFeedback(null), 2500);
+    return () => window.clearTimeout(h);
+  }, [feedback]);
+
+  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");
+  };
+
+  return (
+    <div className="rounded-lg border border-zinc-800 bg-zinc-900/40 p-3">
+      <div className="mb-2 flex items-baseline justify-between">
+        <h3 className="text-xs font-semibold text-zinc-300">투자 메모</h3>
+        <span className="text-[10px] text-zinc-500">
+          {saved ? fmtUpdatedAt(saved.updatedAt) : "비공개 · 브라우저 저장"}
+        </span>
+      </div>
+      <textarea
+        value={text}
+        onChange={(e) => setText(e.target.value.slice(0, NOTE_MAX_LEN))}
+        placeholder="이 종목에 대한 관전 포인트, 진입/청산 근거 등을 적어두세요."
+        rows={4}
+        className="w-full resize-y rounded border border-zinc-800 bg-zinc-950/50 px-2 py-1.5 text-[12px] leading-relaxed text-zinc-100 outline-none focus:border-zinc-600"
+        spellCheck={false}
+      />
+      <div className="mt-2 flex items-center justify-between">
+        <span className="text-[10px] tabular-nums text-zinc-500">
+          {remaining.toLocaleString()}자 남음
+        </span>
+        <div className="flex items-center gap-2">
+          {feedback && (
+            <span className="text-[10px] text-zinc-400" aria-live="polite">
+              {feedback}
+            </span>
+          )}
+          <button
+            type="button"
+            onClick={save}
+            disabled={!dirty}
+            className="rounded-md border border-zinc-700 bg-zinc-800/80 px-2 py-1 text-[11px] text-zinc-100 transition hover:border-zinc-500 disabled:cursor-not-allowed disabled:opacity-40"
+          >
+            저장
+          </button>
+        </div>
+      </div>
+    </div>
+  );
+}

web/lib/notes.ts

@@ -0,0 +1,53 @@
+// 종목별 사용자 메모 (브라우저 한정 — 비로그인 앱).
+//
+// 키: `notes:<code>` → { text, updatedAt }
+// 투자 결정의 근거/관전 포인트를 짧게 적어두는 용도. 다른 사용자에게는 보이지 않음.
+
+const PREFIX = "notes:";
+const MAX_LEN = 2000; // 너무 큰 입력 방어 — localStorage quota 보호.
+
+export type Note = {
+  text: string;
+  updatedAt: string; // ISO
+};
+
+export function readNote(code: string): Note | null {
+  if (typeof window === "undefined") return null;
+  try {
+    const raw = window.localStorage.getItem(`${PREFIX}${code}`);
+    if (!raw) return null;
+    const parsed = JSON.parse(raw) as Partial<Note>;
+    if (typeof parsed.text !== "string" || typeof parsed.updatedAt !== "string") return null;
+    return { text: parsed.text, updatedAt: parsed.updatedAt };
+  } catch {
+    return null;
+  }
+}
+
+export function writeNote(code: string, text: string): Note | null {
+  if (typeof window === "undefined") return null;
+  const trimmed = text.slice(0, MAX_LEN);
+  // 공백만이면 키 자체 제거 → 빈 잔여물 방지.
+  if (!trimmed.trim()) {
+    clearNote(code);
+    return null;
+  }
+  const next: Note = { text: trimmed, updatedAt: new Date().toISOString() };
+  try {
+    window.localStorage.setItem(`${PREFIX}${code}`, JSON.stringify(next));
+    return next;
+  } catch {
+    return null; // quota 등 — UI 가 저장 실패 처리
+  }
+}
+
+export function clearNote(code: string): void {
+  if (typeof window === "undefined") return;
+  try {
+    window.localStorage.removeItem(`${PREFIX}${code}`);
+  } catch {
+    /* ignore */
+  }
+}
+
+export const NOTE_MAX_LEN = MAX_LEN;

web/lib/targets.ts

@@ -24,9 +24,10 @@ export function readTargets(code: string): Targets {
 export function writeTargets(code: string, t: Targets): void {
   if (typeof window === "undefined") return;
   // 둘 다 비어 있으면 키 자체를 지움 → 빈 객체 잔여물 제거.
+  // 양수 유한값만 통과. UI 가 이미 가드하지만 lib 자체도 방어 — 외부에서 직접 호출돼도 안전.
   const cleaned: Targets = {};
-  if (t.target != null && Number.isFinite(t.target)) cleaned.target = t.target;
+  if (t.target != null && Number.isFinite(t.target) && t.target > 0) cleaned.target = t.target;
-  if (t.stop != null && Number.isFinite(t.stop)) cleaned.stop = t.stop;
+  if (t.stop != null && Number.isFinite(t.stop) && t.stop > 0) cleaned.stop = t.stop;
   try {
     if (cleaned.target == null && cleaned.stop == null) {
       window.localStorage.removeItem(`${PREFIX}${code}`);