gitmegabyte/StegoTool ? reverse-engineered prompt

Build me a simple desktop app called StegoTool that lets me hide almost any file inside an image and later pull it back out. I want it to work with PNG, BMP, and JPEG files, and I want it to support both a stealthier mode that stores data in the image and a backup style that appends the file to the end of the image. The app should automatically detect how the hidden file was stored when I try to extract it.

Please include optional password protection using AES 256 GCM so the hidden data can be encrypted before it gets tucked away. I also want a clean modern interface with light and dark mode, plus small toast style messages that confirm when actions succeed or fail.

If needed, check current docs online for any library details. Make it feel like a polished Windows friendly tool where I can run it easily and use it without needing to understand the technical stuff underneath.