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/services/invoices.service.ts

@@ -155,7 +155,7 @@ export {
   previewInvoiceNumber as getNextInvoiceNumberPreview,
 } from "./numbering.service";
 
-function invoiceTotalWithVat(inv: {
+export function invoiceTotalWithVat(inv: {
   apply_vat: boolean | null;
   vat_rate: { toNumber(): number } | null;
   currency: string | null;
@@ -166,14 +166,23 @@ function invoiceTotalWithVat(inv: {
   }>;
 }) {
   const sub = inv.invoice_items.reduce(
-    (s, i) => s + (Number(i.quantity) || 0) * (Number(i.unit_price) || 0),
+    (s, i) =>
+      s +
+      (Number(i.quantity?.toNumber()) || 0) *
+        (Number(i.unit_price?.toNumber()) || 0),
     0,
   );
   const vat = inv.apply_vat
     ? inv.invoice_items.reduce((s, i) => {
-        const base = (Number(i.quantity) || 0) * (Number(i.unit_price) || 0);
+        const base =
+          (Number(i.quantity?.toNumber()) || 0) *
+          (Number(i.unit_price?.toNumber()) || 0);
         const lineVat =
-          base * ((Number(i.vat_rate) || Number(inv.vat_rate) || 21) / 100);
+          base *
+          ((Number(i.vat_rate?.toNumber()) ||
+            Number(inv.vat_rate?.toNumber()) ||
+            21) /
+            100);
         return s + Math.round(lineVat * 100) / 100;
       }, 0)
     : 0;