By far one of my most wide-scope project technical wise.
It's an application to store your tasks & lists (yes I know there are hundreds) with a UI particularly inspired by Actions (previously from Moleskine, now independent).

I really loved the UI/UX of that app, but it didn't feel fully as I wanted, so I built a lists app that worked for me specifically.
The original plan was to have it everywhere: web, iOS, Android, browser extension, Discord bot, and whatever else comes up to your mind.
Then after spending months building the Android app natively with Jetpack Compose new, flagship, full-of-beta-warnings-and-opt-ins, framework, I realized the amount of time that a properly done todo app requires.

I managed to ship an early Android app on the Play Store, back to when I was still on Android, which meant that I also built a backend API in Kotlin.

While the Android app was straight forward as it followed latest Google best-practices, the backend was a combination of services that came out really well:

• ktor rest APIs, fully OpenApi documented
• Postgres as the main DB
• Redis for caching hot data such as authentication (I built my own auth system from scratch too)
• RabbitMQ to create a scalable system of real-time data for the app using websockets
• Sendblue to send auth-related emails (password reset / forgotten flows)
• RevenueCat to manage the user subscription
• Google Cloud Tasks and Firebase for todo reminder's notifications
• BigQuery for statistics

I then setup CircleCI to automatically build the docker image on every commit, and then I have an IaC ArgoCD-compatible repo that deployed all the services needed to my kubernetes cluster (including Postgres and Redis).

After a bit of time though I switched to an iPhone (the 13 mini, 'cause I don't stand big phones, wishing Apple will make another one in the future) so I started building a native iOS app too.
That is now complete and available on the App Store, I built it fully using SwiftUI, and it has:

• extensive integration with the iOS system:
  • Siri support
  • Widgets
  • Shortcuts
  • Share Sheet extension
• offline-read support
• real-time updates via websockets, useful for shared lists
• yes shared lists too

I'm now onto building a minimal web-app, but I don't know how long that'll take as I got a bit burned out with this particular project, it required a lot of effort and seen the current market for those kind of apps, not a lot of feedback from the end users.
Maybe it will be my playground for agentic coding who knows.

I encountered a lot of complex challenges though with this project, from backend stuff such as the auth system to building a UI for the recurring reminders.
I often write about them in my guides.