Is it possible to use the engine without the server ?
Alas, the engine was designed to work with the server-side code automatically generating game metadata (sprite / tile / level / audio / asset definitions) and transform images on-the-fly. It is possible to do all this on the client, but you'd have to write your own game metadata in JSON, and this has not yet been documented. I'll try to find some time to write up documentation for doing this. Thanks for the question!
Tonight I'm going to try the following way: install the server, launch my game via the server and "capture" the assets to serve them as static files later
Oh, if you actually install the server, you can simply "export" your game using "Standalone Publish" (available on the Publish tab after you publish a revision). This will export all your assets, and create a standalone, packaged version of the engine + your game in a ZIP file, ready for hosting. Just keep in mind that this isn't meant for ongoing development, because if your game definition or metadata changes, you'll have to find it in the exported code and modify the JSON manually.