feat(repo): countToday(now?) — KST midnight bucket count

For the F4-C·F cue strengthening surfaces (tray tooltip + Inbox identity
counter), main + renderer need a single source of truth for "오늘 N번
잡아뒀다". Implements `NoteRepository.countToday(now?)` that computes the
half-open UTC interval covering the KST calendar date of `now` and counts
rows whose `created_at` falls inside.

`now` is injectable for deterministic tests across the KST/UTC boundary
(02:00 KST and 23:00 KST land on different UTC dates yet the same / a
different KST day). Four new cases cover empty DB, KST-day filtering,
KST-midnight crossover, and the default-arg branch.

src/main/repository/NoteRepository.ts

@@ -302,6 +302,28 @@ export class NoteRepository {
     return row.c;
   }
 
+  /**
+   * Count notes whose `created_at` falls on the KST calendar date of `now`.
+   * KST = UTC+9. We compute the UTC half-open interval
+   *   [KST-midnight today, KST-midnight tomorrow)
+   * and count rows whose UTC ISO `created_at` lies inside.
+   */
+  countToday(now: Date = new Date()): number {
+    const KST_OFFSET_MS = 9 * 60 * 60 * 1000;
+    const kstNow = new Date(now.getTime() + KST_OFFSET_MS);
+    const kstYear = kstNow.getUTCFullYear();
+    const kstMonth = kstNow.getUTCMonth();
+    const kstDate = kstNow.getUTCDate();
+    const kstMidnightUtc = Date.UTC(kstYear, kstMonth, kstDate) - KST_OFFSET_MS;
+    const nextKstMidnightUtc = kstMidnightUtc + 24 * 60 * 60 * 1000;
+    const startIso = new Date(kstMidnightUtc).toISOString();
+    const endIso = new Date(nextKstMidnightUtc).toISOString();
+    const row = this.db
+      .prepare(`SELECT COUNT(*) AS c FROM notes WHERE created_at >= ? AND created_at < ?`)
+      .get(startIso, endIso) as { c: number };
+    return row.c;
+  }
+
   getAllPendingJobs(): Array<{ noteId: string; attempts: number; nextRunAt: string }> {
     const rows = this.db
       .prepare(`SELECT note_id, attempts, next_run_at FROM pending_jobs`)