What is the ruling on online trading through the "fixed-time" system, which relies on predicting the direction of asset prices (up or down) within a specific timeframe, with a percentage of the amount wagered being profited if the prediction is correct, and the entire amount lost if it is wrong, along with the possibility of sometimes closing the deal before the end of the period?

Gambling is forbidden in all its forms and manifestations. It involves a contest and risk-taking between two parties, where the participant is caught between winning or losing. This applies to the trading described in the question, as it is a forbidden form of gambling (maysir). The Islamic Fiqh Academy has ruled that it is impermissible to buy and sell indices, because it is pure gambling and the selling of an imaginary entity.