fix: use ESM import for mkdirSync instead of require()

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/services/notifications.ts

@@ -1,6 +1,6 @@
 // server/src/services/notifications.ts
 import { createTransport } from 'nodemailer';
-import { readFileSync, writeFileSync, existsSync } from 'fs';
+import { readFileSync, writeFileSync, existsSync, mkdirSync } from 'fs';
 import { resolve } from 'path';
 import { randomUUID } from 'crypto';
 import type { MonitorUser, Notification, UserRole } from '../types/index.js';
@@ -11,7 +11,6 @@ const USERS_FILE = resolve(DATA_DIR, 'users.json');
 const NOTIFICATIONS_FILE = resolve(DATA_DIR, 'notifications.json');
 
 function ensureDataDir() {
-  const { mkdirSync } = require('fs');
   try { mkdirSync(DATA_DIR, { recursive: true }); } catch {}
 }