Translate UI and user-facing messages to Ukrainian (#5)

- All frontend pages, labels, notices and errors; html lang=uk, uk-UA money format
- Brand "Assistant System" in the top bar and page title
- Backend error details returned to the UI (auth, orders, receipts,
  cash registers, Checkbox/CRM/NP errors) and CLI output
- Tests updated for the new messages

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/services/crm/stub_client.py

@@ -12,7 +12,7 @@ _FIXTURE_ORDERS: list[dict] = [
         "RecipientPhone": "+380501112233",
         "RecipientEmail": None,
         "Waybill_Number": "20450123456789",
-        "Notes": "Тестовий заказ",
+        "Notes": "Тестове замовлення",
         "Total": {
             "Cost": "0.00",
             "Quantity": "2",