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/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.stop != null && Number.isFinite(t.stop)) cleaned.stop = t.stop;
+  if (t.target != null && Number.isFinite(t.target) && t.target > 0) cleaned.target = t.target;
+  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}`);