As suggested by ImYubanKRUL (thank you!), I added the ability to save and load your progress in the game!
It will automatically save the upgrades you earned, the coins you possess and the upgrade posts that are partially completed.
Note that it won't save the coins and personal upgrades of other players.
You can choose a save file when you create a new lobby.
The singleplayer save file is separate from the multiplayer ones.
Changes:
- New feature: saving & loading progress
- Nerfed the health of the d e f a u l t c u b e boss