if it would be possible i would highly appreciate if you would be able to considerable and add a feature which allows you to tick a voluntary (optional) fast.

for example:
in the month of shawwal if we keep a total of 6 fasts, (after keeping fasts for ramadan) we get the reward as if we have fasted for an entire year!

source: sahih muslim 1164 a
Abu Ayyub al-Ansari (Allah be pleased with him) reported Allah's Messenger (5) as saying:
He who observed the fast of Ramadan and then followed it with six (fasts) of Shawwal. it would be as if he fasted perpetually.

conclusion: like you have added a section for missed fast, and separate addition of a voluntary fast would be highly amiable as it provides difficulty for me to remember.

‎جزاك الله خيران for reading my request and may allah send you endless of his blessings and shower his mercy on you for making prayer tracking and such easy for everyone ‎ أمين