Add invoice model and validation rules

src/ImportAssist.Cli/Program.cs

@@ -0,0 +1,24 @@
+using ImportAssist.Core;
+
+InvoiceRow[] rows =
+[
+    new("INV-001", "C-100", "125.50", "EUR"),
+    new("INV-002", "",      "89.00",  "EUR"),
+    new("INV-003", "C-300", "abc",    "EUR"),
+    new("INV-004", "C-400", "-10.00", "EUR"),
+    new("",        "",      "0",      "EUR")
+];
+
+foreach (var row in rows)
+{
+    var errors = InvoiceValidator.Validate(row);
+
+    var invoiceId = string.IsNullOrWhiteSpace(row.InvoiceId)
+        ? "(ohne Rechnungsnummer)"
+        : row.InvoiceId;
+
+    Console.WriteLine(
+        errors.Count == 0
+            ? $"{invoiceId}: Gültig"
+            : $"{invoiceId}: {string.Join("; ", errors)}");
+}