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/migrations/20260326_add_invoice_alert_log/migration.sql

@@ -0,0 +1,9 @@
+CREATE TABLE `invoice_alert_log` (
+  `id` INT NOT NULL AUTO_INCREMENT,
+  `invoice_type` VARCHAR(20) NOT NULL,
+  `invoice_id` INT NOT NULL,
+  `alert_type` VARCHAR(20) NOT NULL,
+  `sent_at` DATETIME(0) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  PRIMARY KEY (`id`),
+  UNIQUE KEY `invoice_type_invoice_id_alert_type` (`invoice_type`, `invoice_id`, `alert_type`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;