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/schemas/offers.schema.ts

@@ -5,14 +5,14 @@ const QuotationItemSchema = z.object({
   item_description: z.string().nullish(),
   quantity: z
     .union([z.number(), z.string()])
-    .transform((v) => Number(v) || 1)
+    .transform((v) => Number(v))
     .refine((v) => !Number.isNaN(v), { message: "Must be a valid number" })
     .optional()
     .default(1),
   unit: z.string().nullish(),
   unit_price: z
     .union([z.number(), z.string()])
-    .transform((v) => Number(v) || 0)
+    .transform((v) => Number(v))
     .refine((v) => !Number.isNaN(v), { message: "Must be a valid number" })
     .optional()
     .default(0),

src/schemas/orders.schema.ts

@@ -5,14 +5,14 @@ const OrderItemSchema = z.object({
   item_description: z.string().nullish(),
   quantity: z
     .union([z.number(), z.string()])
-    .transform((v) => Number(v) || 1)
+    .transform((v) => Number(v))
     .refine((v) => !Number.isNaN(v), { message: "Must be a valid number" })
     .optional()
     .default(1),
   unit: z.string().nullish(),
   unit_price: z
     .union([z.number(), z.string()])
-    .transform((v) => Number(v) || 0)
+    .transform((v) => Number(v))
     .refine((v) => !Number.isNaN(v), { message: "Must be a valid number" })
     .optional()
     .default(0),