Yes, this is a very annoying issue I am unsure about how to fix at the moment: the game currently does not have an auto-save function, so the only way to save is by exiting the game from the pause menu. On Desktop, closing the app itself will also save automatically. On browser though, reloading the tab/window does not allow me to "react" to it, well, because the browser will basically just kill the game.
Maybe a good way to solve this is to auto-save whenever the player does something important (like mining or forging), however, that might impact performance (as saving may take several seconds)