Great aesthetic, cool idea, the Morris brothers are super skilled at their craft.
Two pieces of feedback:
1. I would find the UI to be more comfortable if I were able to swap the bubbles' positions by clicking and dragging one bubble to another's location. Same goes for bubbles in the player's inventory
2. Some kind of visual flair or instruction around the "ready" button when all bubbles have been placed could improve clarity. Not vital but I've come to expect these sort of UI elements in turn-based roguelikes