where does twine store its project files
That depends on which release of the Twine 2.x application you are using, and which Operating System you are using it on.
- Using one of the Web-browser based releases (Hosted or Downloaded and run locally)
These releases store your project’s meta data within the web-browser’s own Local Storage cache, and where that cache is physically stored on your local hard-drive depends of on the brand of web-browser you are using.
- Using one of the Desktop (installable) releases.
The folder (on you local hard-drive) that this release uses depends on the Operating System you are using. You can use the application’s Twine > Show Library menu item to determine which folder is being used, this folder is generally within the area that your OS allocates to the storage of “User” related files.
It is possible to use “directory based” symbolic link to replace the above mentioned folder with one that references a different location on your local hard-drive. You could then use this different location as a target of a file sharing application like the one you mentioned.
twine could do with some kind of backup option
More recent versions of the Desktop release do include functionality that automatically creates “backups” of the project(s) you are working on.