Really enjoyable game. I really enjoyed how you used time as a mechanic in this game. When I remember back to the arcade shooters I played as a kid, one easy way to stay alive was just to dodge the dumb ships that only go in a straight line and save your bombs for the bosses. However in this game you are heavily incentivized to fight, which was not something I was used to.
The graphics are simple but work very well for the game. Each ship's design was distinct enough so that they were easily recognizable (so you know which ones you have to shoot to get the most time).
I just had two small gripes:
1. It seems as though your bullets either bounce off or phase through the vertices of the ships. Again this was different from what I was used to because in older games of this genre, even hitting a ship slightly was the same as hitting it dead center. This made the game a lot harder, but I understand if it's a deliberate design decision.
2. Having the health clock sit right underneath the ship made me think that your ship had a larger ship than it did. For instance I was trying to grab the +5 second clocks by touching it with my clock (instead of my ship) and was frustrated that it didn't work. Maybe putting the health clock at the top or in the corner would've been less confusing UI wise.
Overall a very well designed game!
(By the way, I looked through your code and I was surprised that you had a client-server architecture for a single player game. I didn't see any leaderboards when I was playing the game. Just curious as to why you chose this architecture. I think if I was making the same game I would've just done the frontend.)