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.

tests/unit/NoteRepository.test.ts

@@ -175,4 +175,43 @@ describe('NoteRepository', () => {
     const note = repo.findById(id)!;
     expect(note.dueDate).toBeNull();
   });
+
+  it('countToday returns 0 for empty DB', () => {
+    expect(repo.countToday(new Date('2026-04-26T12:00:00Z'))).toBe(0);
+  });
+
+  it('countToday counts notes created on the KST date of "now"', () => {
+    // now = 2026-04-26 14:00 KST (= 2026-04-26T05:00:00Z UTC).
+    // a, b: 2026-04-26 KST → counted.
+    // c: 2026-04-25 KST → excluded.
+    db.prepare(
+      `INSERT INTO notes (id, raw_text, ai_status, created_at, updated_at)
+       VALUES (?, 'x', 'pending', ?, ?)`
+    ).run('a', '2026-04-25T17:00:00Z', '2026-04-25T17:00:00Z'); // 04-26 02:00 KST
+    db.prepare(
+      `INSERT INTO notes (id, raw_text, ai_status, created_at, updated_at)
+       VALUES (?, 'x', 'pending', ?, ?)`
+    ).run('b', '2026-04-25T18:00:00Z', '2026-04-25T18:00:00Z'); // 04-26 03:00 KST
+    db.prepare(
+      `INSERT INTO notes (id, raw_text, ai_status, created_at, updated_at)
+       VALUES (?, 'x', 'pending', ?, ?)`
+    ).run('c', '2026-04-25T14:00:00Z', '2026-04-25T14:00:00Z'); // 04-25 23:00 KST
+    expect(repo.countToday(new Date('2026-04-26T05:00:00Z'))).toBe(2);
+  });
+
+  it('countToday handles KST midnight crossover', () => {
+    // now = 2026-04-26 14:00 KST. A note at 2026-04-26T23:30Z = 2026-04-27 08:30 KST
+    // belongs to "tomorrow" (KST), so MUST NOT be counted as "today".
+    db.prepare(
+      `INSERT INTO notes (id, raw_text, ai_status, created_at, updated_at)
+       VALUES (?, 'x', 'pending', ?, ?)`
+    ).run('a', '2026-04-26T23:30:00Z', '2026-04-26T23:30:00Z');
+    expect(repo.countToday(new Date('2026-04-26T05:00:00Z'))).toBe(0);
+  });
+
+  it('countToday default arg uses Date.now()', () => {
+    const n = repo.countToday();
+    expect(typeof n).toBe('number');
+    expect(n).toBeGreaterThanOrEqual(0);
+  });
 });