Is having something one usually has before a fast mutar to have on shabbos for the Sunday fast if he doesn’t SAY that’s the reason?
Regular food is permitted, because it is a normal thing to do on Shabbos, but the person shouldn’t say that it is in order to prepare for the fast. Regarding taking pills to help fast easier, it is controversial, therefore the best thing is to mix it into food, (preferably before shabbos), and eat the food on shabbos.
Have an easy fast
M:B 290-4, Piskei Teshivos 290-4. Regarding taking Kali Tzom etc. to make the fast easier, there are essentially two issues, hachana for after Shabbos and refuah. Regarding hachana the poskim that permit it say that it isn’t hachana because it is not noticeable that it is being done for after Shabbos, (and of course the person should not say that this is why they are doing it), see Shabbos Shleimo 10-12, Orchos Shabbos 22-186 ftnt. 284, Minchas Asher 2-38( 4), Piskei Halachos (Bein Hamitzrim) 16-17. However, see Hishmas Avrohom 556-1, and Asherei Ish O:CH 2 36-43 from R’ Eliyashiv that it is hachana. Regarding refuah see sources. Also see M’bais Halevi Inyonei Bein Hamitzorim pg. 43-15, that it should be mixed into food, preferably before Shabbos. Also see Shvus Yitzchok 17pg. 82 and Chut Shani -Shabbos 4 pg. 210.