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>

package.json

@@ -43,6 +43,7 @@
     "framer-motion": "^12.38.0",
     "hi-base32": "^0.5.1",
     "jsonwebtoken": "^9.0.3",
+    "node-cron": "^4.2.1",
     "nodemailer": "^8.0.2",
     "otpauth": "^9.5.0",
     "prisma": "^6.19.2",
@@ -61,6 +62,7 @@
     "@types/jsonwebtoken": "^9.0.10",
     "@types/mysql": "^2.15.27",
     "@types/node": "^25.5.0",
+    "@types/node-cron": "^3.0.11",
     "@types/nodemailer": "^7.0.11",
     "@types/qrcode": "^1.5.6",
     "@types/react": "^19.2.14",