feat: invoice due date email alerts, add favicon

- Daily cron (8:00 AM) checks created and received invoices
- Alerts 3 days before due date and on due date
- Summary email to INVOICE_ALERT_EMAIL with grouped tables
- Tracks sent alerts in invoice_alert_log to prevent duplicates
- node-cron scheduler runs inside the app process
- Favicon files copied from PHP project

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

src/config/env.ts

@@ -65,6 +65,7 @@ export const config = {
     smtpFrom: process.env.SMTP_FROM || "",
     smtpFromName: process.env.SMTP_FROM_NAME || "BOHA Automation",
     leaveNotify: process.env.LEAVE_NOTIFY_EMAIL || "",
+    invoiceAlert: process.env.INVOICE_ALERT_EMAIL || "",
   },
 
   appUrl: process.env.APP_URL || "",