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/client.py

@@ -63,7 +63,7 @@ def get_checkbox_client() -> CheckboxClient:
     """Один экземпляр на процесс: внутри — кэш токенов кассиров."""
     if settings.checkbox_use_stub:
         if settings.is_production:
-            raise RuntimeError("CHECKBOX_USE_STUB=true запрещён в production")
+            raise RuntimeError("CHECKBOX_USE_STUB=true заборонено в production")
         from app.services.checkbox.stub_client import StubCheckboxClient
 
         return StubCheckboxClient(auto_complete_after=2)

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

backend/app/services/checkbox/stub_client.py

@@ -32,12 +32,12 @@ class StubCheckboxClient:
 
     async def sign_in(self, creds: CheckboxCredentials) -> None:
         if not creds.license_key or not creds.pin_code:
-            raise CheckboxError("Вход кассира не удался: пустой ключ или PIN")
+            raise CheckboxError("Вхід касира не вдався: порожній ключ або PIN")
 
     async def create_ettn(self, creds: CheckboxCredentials, body: dict[str, Any]) -> EttnOut:
         waybill = body["receipt_body"]["payments"][0]["ettn"]
         if await self.find_ettn(creds, waybill) is not None:
-            raise CheckboxError(f"ЕТТН {waybill} уже привязана к чеку")
+            raise CheckboxError(f"ЕТТН {waybill} вже прив'язана до чека")
         ettn = EttnOut(
             id=str(uuid.uuid4()),
             status=EttnStatus.CREATED,
@@ -50,7 +50,7 @@ class StubCheckboxClient:
 
     async def get_ettn(self, creds: CheckboxCredentials, ettn_id: str) -> EttnOut:
         if ettn_id not in self.orders:
-            raise CheckboxError(f"ЕТТН-чек {ettn_id} не найден")
+            raise CheckboxError(f"ЕТТН-чек {ettn_id} не знайдено")
         self._polls[ettn_id] = self._polls.get(ettn_id, 0) + 1
         if (
             self.auto_complete_after is not None