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-lock.json

@@ -28,6 +28,7 @@
         "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",
@@ -46,6 +47,7 @@
         "@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",
@@ -1547,6 +1549,13 @@
         "undici-types": "~7.18.0"
       }
     },
+    "node_modules/@types/node-cron": {
+      "version": "3.0.11",
+      "resolved": "https://registry.npmjs.org/@types/node-cron/-/node-cron-3.0.11.tgz",
+      "integrity": "sha512-0ikrnug3/IyneSHqCBeslAhlK2aBfYek1fGo4bP4QnZPmiqSGRK+Oy7ZMisLWkesffJvQ1cqAcBnJC+8+nxIAg==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/@types/nodemailer": {
       "version": "7.0.11",
       "resolved": "https://registry.npmjs.org/@types/nodemailer/-/nodemailer-7.0.11.tgz",
@@ -4232,6 +4241,15 @@
         "node": ">= 0.4.0"
       }
     },
+    "node_modules/node-cron": {
+      "version": "4.2.1",
+      "resolved": "https://registry.npmjs.org/node-cron/-/node-cron-4.2.1.tgz",
+      "integrity": "sha512-lgimEHPE/QDgFlywTd8yTR61ptugX3Qer29efeyWw2rv259HtGBNn1vZVmp8lB9uo9wC0t/AT4iGqXxia+CJFg==",
+      "license": "ISC",
+      "engines": {
+        "node": ">=6.0.0"
+      }
+    },
     "node_modules/node-fetch-native": {
       "version": "1.6.7",
       "resolved": "https://registry.npmjs.org/node-fetch-native/-/node-fetch-native-1.6.7.tgz",