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/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