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>

src/main/repository/NoteRepository.ts

@@ -1,7 +1,7 @@
 import type Database from 'better-sqlite3';
 import { v7 as uuidv7, v4 as uuidv4 } from 'uuid';
 import type { Note, NoteMedia, NoteTag } from '@shared/types';
-import { todayInKstString } from '../util/kstDate.js';
+import { kstTodayIso } from '../../shared/util/kstDate.js';
 
 export interface CreateNoteInput { rawText: string; }
 
@@ -601,7 +601,7 @@ export class NoteRepository {
    * Caller may inject `now` for testability; defaults to `new Date()`.
    */
   findExpiredCandidates(now: Date = new Date()): Note[] {
-    const today = todayInKstString(now);
+    const today = kstTodayIso(now);
     const rows = this.db
       .prepare(
         `SELECT * FROM notes