It would be nice to have a mobile app that can run on the desktop/pinephone.
QML/c++ works well cross platform from my experience.
Home Assistant already has a “Progressive Web App” that works on any platform that supports PWAs. On Chrome, there should be a “plus” button in your URL bar to “install” it.
The apps on iOS and Android are 95% just a PWA, with the addition native OS functionality like sensors, location tracking, notifications etc.
If you want those added features, the reality is that the apps require upkeep and the market share of something like Pinephone versus Android/iOS is magnitudes smaller. There isn’t even (at the time of writing) a Windows version because it would require writing a brand new client - macOS is in beta, but only because it repurposed the iOS code.
Of course HA is open source, so you are more than welcome to make it yourself 
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