test: add regression tests for Critical+High FLAWS_REPORT fixes

- Tests caught 2 real bugs:
  - Zod NaN bypass in orders/offers schemas (Number(v) || fallback)
  - invoiceTotalWithVat using Number() on { toNumber() } objects
- 7 new test files covering auth, env, exchange rates, NAS paths,
  schema NaN rejection, invoice VAT calculation, customer validation
- 45 tests passing, build clean

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/__tests__/invoices.service.test.ts

@@ -0,0 +1,55 @@
+import { describe, it, expect } from "vitest";
+import { invoiceTotalWithVat } from "../services/invoices.service";
+
+describe("invoiceTotalWithVat", () => {
+  it("calculates subtotal without VAT when apply_vat is false", () => {
+    const inv = {
+      apply_vat: false,
+      vat_rate: { toNumber: () => 21 },
+      currency: "CZK",
+      invoice_items: [
+        {
+          quantity: { toNumber: () => 2 },
+          unit_price: { toNumber: () => 100 },
+          vat_rate: { toNumber: () => 21 },
+        },
+      ],
+    };
+    expect(invoiceTotalWithVat(inv)).toBe(200);
+  });
+
+  it("rounds each line VAT to 2 decimals before accumulation", () => {
+    // 3 items @ 33.33 with 21% VAT
+    // Line VAT = 33.33 * 0.21 = 6.9993 -> rounded to 7.00
+    // Total VAT = 7.00 * 3 = 21.00
+    // Subtotal = 33.33 * 3 = 99.99
+    // Total = 99.99 + 21.00 = 120.99
+    const inv = {
+      apply_vat: true,
+      vat_rate: { toNumber: () => 21 },
+      currency: "CZK",
+      invoice_items: Array.from({ length: 3 }, () => ({
+        quantity: { toNumber: () => 1 },
+        unit_price: { toNumber: () => 33.33 },
+        vat_rate: { toNumber: () => 21 },
+      })),
+    };
+    expect(invoiceTotalWithVat(inv)).toBe(120.99);
+  });
+
+  it("handles null quantity and unit_price gracefully", () => {
+    const inv = {
+      apply_vat: true,
+      vat_rate: { toNumber: () => 21 },
+      currency: "CZK",
+      invoice_items: [
+        {
+          quantity: { toNumber: () => 0 },
+          unit_price: { toNumber: () => 0 },
+          vat_rate: { toNumber: () => 21 },
+        },
+      ],
+    };
+    expect(invoiceTotalWithVat(inv)).toBe(0);
+  });
+});