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/deps.py

@@ -25,7 +25,7 @@ SessionDep = Annotated[AsyncSession, Depends(get_session)]
 
 _UNAUTHORIZED = HTTPException(
     status_code=status.HTTP_401_UNAUTHORIZED,
-    detail="Требуется аутентификация",
+    detail="Потрібна автентифікація",
     headers={"WWW-Authenticate": "Bearer"},
 )
 
@@ -76,7 +76,7 @@ def require_roles(*roles: UserRole) -> Callable[..., Coroutine[Any, Any, User]]:
         if user.role not in allowed:
             raise HTTPException(
                 status_code=status.HTTP_403_FORBIDDEN,
-                detail="Недостаточно прав для этого действия",
+                detail="Недостатньо прав для цієї дії",
             )
         return user