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>

prisma/schema.prisma

@@ -601,6 +601,16 @@ enum leave_requests_status {
   cancelled
 }
 
+model invoice_alert_log {
+  id           Int      @id @default(autoincrement())
+  invoice_type String   @db.VarChar(20) // "created" or "received"
+  invoice_id   Int
+  alert_type   String   @db.VarChar(20) // "3days" or "due"
+  sent_at      DateTime @default(now()) @db.DateTime(0)
+
+  @@unique([invoice_type, invoice_id, alert_type])
+}
+
 enum received_invoices_status {
   unpaid
   paid