Tadpole

For Android. Your diary is kept on your phone and nowhere else, and it works with no internet. Open the downloaded file and let Android install it (it will ask permission for your browser the first time, because the app is not on a store). Then add the widget: press and hold an empty spot on your home screen, choose Widgets, find Tadpole, and drag it out. It has buttons for a meal, stool, how your head feels, and a box to type in.

Optional. It keeps a copy of your phone's diary on your computer, shows it on a big screen, and holds documents next to it, like a diet plan. It needs Python 3.10 or newer. Unzip the folder, run python3 start.py (on Windows py start.py), and your browser opens to a pairing page. Scan the code with your phone's camera. They connect over your home Wi-Fi; nothing goes through the internet. If you use Tailscale, the pairing code includes your Tailscale address too, so your phone can sync from anywhere.

The same app, running in this browser tab. The demo is filled with made-up entries so you can look around. The other one starts empty and keeps your entries in this browser only, which works on an iPhone too; there is no widget outside the Android app.