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__/customers.schema.test.ts

@@ -0,0 +1,19 @@
+import { describe, it, expect } from "vitest";
+import { UpdateCustomerSchema } from "../schemas/customers.schema";
+
+describe("UpdateCustomerSchema", () => {
+  it("rejects empty name", () => {
+    const result = UpdateCustomerSchema.safeParse({ name: "" });
+    expect(result.success).toBe(false);
+  });
+
+  it("accepts valid name", () => {
+    const result = UpdateCustomerSchema.safeParse({ name: "Acme Corp" });
+    expect(result.success).toBe(true);
+  });
+
+  it("accepts partial updates without name", () => {
+    const result = UpdateCustomerSchema.safeParse({ street: "Main St" });
+    expect(result.success).toBe(true);
+  });
+});