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/server.ts

@@ -177,6 +177,21 @@ async function start() {
     });
   }
 
+  // --- Invoice alert cron (daily at 8:00 AM) ---
+  if (config.email.invoiceAlert) {
+    const cron = await import("node-cron");
+    cron.default.schedule("0 8 * * *", async () => {
+      try {
+        const { checkInvoiceAlerts } =
+          await import("./services/invoice-alerts");
+        await checkInvoiceAlerts();
+      } catch (err) {
+        app.log.error(err, "Invoice alert cron failed");
+      }
+    });
+    console.log("Invoice alert cron scheduled (daily 8:00)");
+  }
+
   // --- Start ---
   const port = config.isProduction ? config.port : 3000;
   try {