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/checkbox/http_client.py

@@ -57,7 +57,7 @@ def _error_message(response: httpx.Response) -> str:
         # {"code": "third_party.*", "message": "Internal Server Error"} —
         # без кода кассир видит только бесполезное «Internal Server Error».
         if code := data.get("code"):
-            message = f"{message or 'Ошибка'} ({code})"
+            message = f"{message or 'Помилка'} ({code})"
         if isinstance(detail, list) and detail:
             parts = [
                 f"{'.'.join(str(p) for p in item.get('loc', []))}: {item.get('msg')}"
@@ -130,7 +130,7 @@ class HttpCheckboxClient:
                         method, path, headers=headers, json=json, params=params
                     )
             except httpx.HTTPError as exc:
-                raise CheckboxUnavailableError(f"Checkbox недоступен: {exc!r}") from exc
+                raise CheckboxUnavailableError(f"Checkbox недоступний: {exc!r}") from exc
         if error := _transient_error(response):
             raise error
         return response
@@ -155,7 +155,7 @@ class HttpCheckboxClient:
             json={"pin_code": creds.pin_code},
         )
         if response.status_code != 200:
-            raise CheckboxError(f"Вход кассира не удался: {_error_message(response)}")
+            raise CheckboxError(f"Вхід касира не вдався: {_error_message(response)}")
         token = response.json()["access_token"]
         self._tokens[creds.cash_register_id] = token
         return token