[1.3.0] Add discount codes for students and people in financial need

Because I want as many people as possible to benefit from Budget Flow, and budgets can sometimes be tight, there should be a discount code for anyone who cannot afford to buy / subscribe to the app at full price (e.g. students or those struggling financially). By creating a discount code, Budget Flow can be made more accessible to anyone in a difficult financial situation.

On the one hand, this helps to educate young people about managing their finances, and on the other hand, it may also help people in financial need to get out of that situation by using the app.

Unfortunately, since it is not possible to generate promo codes for the lifetime version of the app from the App Store, the discount only applies to the annual subscription. Nevertheless, I think it will help a lot of students or people in a difficult financial situation not to have to pay the full price.