[Ygorl]

You need to hit the game from the "network" tab when a new turn hosts. If you do "end turn" while connected, it uploads your turn and you don't have to worry. If you do one of the "quit" options, you will still have the turn downloaded onto your computer. Anything you do from the "play an existing game" option will be saved on your computer, and uploaded to the server the next time you connect.
So, in summary, two things:
1) you must be connected to the server (via "network" option) to upload a turn
2) your turn is automatically uploaded (and marked as "done" for the purposes of quickhosting) as soon as you connect (after the connection in which you downloaded the turn).
You can play your turns in one sitting, connected via "network", and finish with "end turn". You can also work on the turn locally, via "play an existing game", and your changes will be automatically uploaded next time you connect (via "network").
I sort of repeated myself a lot, but it's not very intuitive and I am writing stream-of-consciousness. I hope it made things clear for everyone...