MovEaxEax/mp4-fps-hack ? reverse-engineered prompt

Build me a simple Windows app that takes a video file and rewrites it so it looks like one frame rate on paper, but plays at a different speed on a phone player. I want to pick an input video, choose an output mp4, set a target fps, and optionally switch between a few patching modes and safe codec settings if the first try does not work.

It should work from the command line, be easy to use with a default 30 fps mode, and support common video files like mp4, avi, and mov. Please make it rely on ffmpeg being installed on the system, and show a helpful error if it is missing. If you need to look up current docs online for anything, go ahead.

This is okay if it is experimental, but I want the app to clearly explain what it is doing and warn that some files may have sync issues or not work with every player.