perarnia/fSpy ? reverse-engineered prompt

Build me a cross platform desktop app that helps match a 3D camera to a still image. I want to be able to load a photo, line up perspective guides, and get the camera settings needed to recreate that shot in another app like Blender.

Make it feel simple and fast to use, with a clean interface for placing and adjusting the guides directly on the image. It should let me save a project, reopen it later, and export or copy the computed camera parameters in a way that other tools can use. If there are already docs or an existing file format for this project, follow that so it stays compatible.

Use current docs online if you need to, and keep the app working on desktop across platforms.