refactor(v026): #3+#19+#34 KST helper 통합 → src/shared/util/kstDate.ts

기존 src/main/util/kstDate.ts (2 함수) 를 shared 로 이동 + kstTodayAsDate 추가.
main + renderer 양쪽 import 가능. 6 callsite 통합:
- NoteRepository.findExpiredCandidates (todayInKstString → kstTodayIso)
- TelemetryService.todayKstIso (inline 제거)
- telemetryStats.kstDate (inline 제거)
- AiWorker.todayKstAsDate / todayKstAsIso (inline 제거)
- store.snoozeExpired + snoozeRecall (inline 제거 → nextKstMidnightMs)

API: kstTodayIso(now) / nextKstMidnightMs(now) / kstTodayAsDate(now)
+ KST_OFFSET_MS, DAY_MS 상수 export.

단위 +4 cases (boundary, format, midnight, asDate). 418 → 422.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/kstDate.test.ts

@@ -1,18 +1,29 @@
 import { describe, it, expect } from 'vitest';
-import { todayInKstString, nextKstMidnightMs } from '@main/util/kstDate.js';
+import { kstTodayIso, nextKstMidnightMs, kstTodayAsDate } from '@shared/util/kstDate.js';
 
-describe('todayInKstString', () => {
+describe('kstTodayIso', () => {
   it('returns KST calendar date as YYYY-MM-DD', () => {
     // 2026-05-01 12:00 UTC = 2026-05-01 21:00 KST
-    expect(todayInKstString(new Date('2026-05-01T12:00:00Z'))).toBe('2026-05-01');
+    expect(kstTodayIso(new Date('2026-05-01T12:00:00Z'))).toBe('2026-05-01');
   });
 
   it('handles UTC→KST date rollover (UTC 23:30 → KST next day 08:30)', () => {
-    expect(todayInKstString(new Date('2026-05-01T23:30:00Z'))).toBe('2026-05-02');
+    expect(kstTodayIso(new Date('2026-05-01T23:30:00Z'))).toBe('2026-05-02');
   });
 
   it('handles KST midnight exactly (UTC 15:00 = KST 00:00 next day)', () => {
-    expect(todayInKstString(new Date('2026-05-01T15:00:00Z'))).toBe('2026-05-02');
+    expect(kstTodayIso(new Date('2026-05-01T15:00:00Z'))).toBe('2026-05-02');
+  });
+
+  it('boundary — UTC 14:59:59 still KST 23:59:59 same day', () => {
+    // KST 5/4 23:59:59 = UTC 5/4 14:59:59
+    const utcDate = new Date('2026-05-04T14:59:59Z');
+    expect(kstTodayIso(utcDate)).toBe('2026-05-04');
+  });
+
+  it('KST 5/5 00:30 (UTC 5/4 15:30) returns 2026-05-05', () => {
+    const utcDate = new Date('2026-05-04T15:30:00Z');
+    expect(kstTodayIso(utcDate)).toBe('2026-05-05');
   });
 });
 
@@ -34,4 +45,19 @@ describe('nextKstMidnightMs', () => {
     expect(next - now).toBeGreaterThan(23 * 60 * 60 * 1000);
     expect(next - now).toBeLessThan(24 * 60 * 60 * 1000);
   });
+
+  it('KST 5/5 00:30 → next KST midnight = 5/6 00:00 KST = 5/5 15:00 UTC', () => {
+    const utcMs = new Date('2026-05-04T15:30:00Z').getTime();
+    const next = nextKstMidnightMs(utcMs);
+    expect(new Date(next).toISOString()).toBe('2026-05-05T15:00:00.000Z');
+  });
+});
+
+describe('kstTodayAsDate', () => {
+  it('returns UTC Date at KST 00:00', () => {
+    // KST 5/5 00:30 → KST 5/5 00:00 = UTC 5/4 15:00
+    const utcDate = new Date('2026-05-04T15:30:00Z');
+    const result = kstTodayAsDate(utcDate);
+    expect(result.toISOString()).toBe('2026-05-05T00:00:00.000Z');
+  });
 });