buttercup/buttercup-desktop ? reverse-engineered prompt

Build me a cross platform desktop password manager for Mac, Linux, and Windows that lets people safely store passwords, logins, and other secret notes in encrypted vault files. I want a clean app where someone can create a vault, unlock it with a strong master password, add and edit entries, and keep their data protected on disk. The app should support opening and saving vaults from local files, and also work with online storage like Dropbox, Google Drive, and WebDAV so people can keep their vault synced. It should feel simple and trustworthy, with a modern desktop interface, offline access to cached vaults, and sensible app settings for things like startup and update checks. Use the existing project structure and make it work as a real Electron desktop app. If you need to check current docs or best practices for any library pieces, look them up online before wiring things up.