feat(targets): price target/stop simulator with chart overlay lines

- New lib/targets.ts: per-code localStorage (key `targets:<code>`) with
  read/write/clear; rejects non-positive/non-finite values.
- New PriceTargets sidebar panel: 목표가/손절가 inputs, live %-from-current
  preview (rose=up, sky=down), save/clear actions.
- StockChart accepts optional `targets` prop and draws dashed horizontal
  price lines on the candle series (rose=target, sky=stop) with axis labels.
- Page loads saved targets on mount/code-change and re-renders chart on save.

Also fixes view-counter dedupe caveat from reviewer: split into
wasRecordedToday() + markRecorded(); the page now marks localStorage only
after POST /api/views succeeds, so a failed POST stays retryable on the
next visit instead of being silently swallowed for the day.

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

web/lib/targets.ts

@@ -0,0 +1,49 @@
+// 종목별 목표가/손절가 저장 (브라우저 한정 — 비로그인 앱).
+//
+// 키: `targets:<code>` → { target?: number, stop?: number, savedAt: ISO }
+// 차트 오버레이 라인과 손익 미리보기에 사용. 알림 없음 (그건 AlertsPanel 담당).
+
+const PREFIX = "targets:";
+
+export type Targets = {
+  target?: number | null; // 익절가 (선택)
+  stop?: number | null;   // 손절가 (선택)
+  savedAt?: string;
+};
+
+export function readTargets(code: string): Targets {
+  if (typeof window === "undefined") return {};
+  try {
+    const raw = window.localStorage.getItem(`${PREFIX}${code}`);
+    return raw ? (JSON.parse(raw) as Targets) : {};
+  } catch {
+    return {};
+  }
+}
+
+export function writeTargets(code: string, t: Targets): void {
+  if (typeof window === "undefined") return;
+  // 둘 다 비어 있으면 키 자체를 지움 → 빈 객체 잔여물 제거.
+  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;
+  try {
+    if (cleaned.target == null && cleaned.stop == null) {
+      window.localStorage.removeItem(`${PREFIX}${code}`);
+      return;
+    }
+    cleaned.savedAt = new Date().toISOString();
+    window.localStorage.setItem(`${PREFIX}${code}`, JSON.stringify(cleaned));
+  } catch {
+    /* quota 초과 등 — 무시 */
+  }
+}
+
+export function clearTargets(code: string): void {
+  if (typeof window === "undefined") return;
+  try {
+    window.localStorage.removeItem(`${PREFIX}${code}`);
+  } catch {
+    /* ignore */
+  }
+}