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/api/v1/cash_registers.py

@@ -44,7 +44,7 @@ async def _ensure_np_key_unique(
         if same:
             raise HTTPException(
                 status_code=status.HTTP_409_CONFLICT,
-                detail=f"Этот ключ Новой Почты уже привязан к кассе «{register.name}»",
+                detail=f"Цей ключ Нової Пошти вже прив'язаний до каси «{register.name}»",
             )
 
 
@@ -101,7 +101,7 @@ async def update_cash_register(
 ) -> CashRegisterOut:
     register = await session.get(CashRegister, register_id)
     if register is None:
-        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Касса не найдена")
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Касу не знайдено")
 
     changes = payload.model_dump(exclude_unset=True)
     if "license_key" in changes:
@@ -139,7 +139,7 @@ async def check_cash_register(
     """Пробный вход кассира в Checkbox — проверка ключа лицензии и PIN."""
     register = await session.get(CashRegister, register_id)
     if register is None:
-        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Касса не найдена")
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Касу не знайдено")
     try:
         await checkbox.sign_in(credentials(register))
     except (CheckboxError, crypto.DecryptionError) as exc: