feat(notify): NotificationService with 4 rotating reward copies

Task 15 of the slice plan. Strategy §4.1 immediate-reward
toast. celebrate(noteId) deterministically picks one of the
4 reward copies via SHA-256(noteId)[0] % 4, then forwards to
the injected send() callback (which Task 30 wires to a real
electron Notification). Skips silently when isSupported() is
false (denied OS permission), and swallows send() errors so
that capture path never fails because of a notification quirk.

Verification: `npx vitest run tests/unit/NotificationService.test.ts`
3 passed.

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

src/main/services/NotificationService.ts

@@ -0,0 +1,33 @@
+import { createHash } from 'node:crypto';
+
+export const REWARD_COPIES = [
+  '이 생각은 이제 Inkling이 들고 있습니다.',
+  '나중에 찾을 수 있게 보관했습니다.',
+  '방금 하나의 업무 기억을 구출했습니다.',
+  '기록 완료. 이제 잊어도 됩니다.'
+] as const;
+
+export interface NotificationDeps {
+  isSupported: () => boolean;
+  send: (body: string) => void;
+}
+
+export class NotificationService {
+  constructor(private deps: NotificationDeps) {}
+
+  celebrate(noteId: string): void {
+    if (!this.deps.isSupported()) return;
+    const idx = this.pick(noteId);
+    const body = REWARD_COPIES[idx]!;
+    try {
+      this.deps.send(body);
+    } catch {
+      // Swallow notification errors — capture must not fail because of toast.
+    }
+  }
+
+  private pick(noteId: string): number {
+    const hash = createHash('sha256').update(noteId).digest();
+    return hash[0]! % REWARD_COPIES.length;
+  }
+}

tests/unit/NotificationService.test.ts

@@ -0,0 +1,41 @@
+import { describe, it, expect } from 'vitest';
+import { NotificationService, REWARD_COPIES } from '@main/services/NotificationService.js';
+
+describe('NotificationService', () => {
+  it('rotates copy deterministically by noteId hash', () => {
+    const fired: string[] = [];
+    const svc = new NotificationService({
+      isSupported: () => true,
+      send: (body) => { fired.push(body); }
+    });
+    const id1 = '00000000-0000-7000-8000-000000000001';
+    svc.celebrate(id1);
+    svc.celebrate(id1);
+    expect(fired).toHaveLength(2);
+    expect(fired[0]).toBe(fired[1]);
+    expect(REWARD_COPIES).toContain(fired[0]);
+  });
+
+  it('different ids select different (eventually all 4)', () => {
+    const fired: string[] = [];
+    const svc = new NotificationService({
+      isSupported: () => true,
+      send: (body) => { fired.push(body); }
+    });
+    for (let i = 0; i < 32; i++) {
+      svc.celebrate(`id-${i}`);
+    }
+    const distinct = new Set(fired);
+    expect(distinct.size).toBe(REWARD_COPIES.length);
+  });
+
+  it('skips silently when notifications unsupported', () => {
+    const fired: string[] = [];
+    const svc = new NotificationService({
+      isSupported: () => false,
+      send: (body) => { fired.push(body); }
+    });
+    svc.celebrate('id-1');
+    expect(fired).toHaveLength(0);
+  });
+});