This is awesome, and your post about coming back to front-end after the jQuery era really resonates. It’s a whole new world! Major props for diving into web components and shipping this.
One small suggestion that might be a fun addition for a power user: keyboard shortcuts. Being able to hit Spacebar to start/stop the most recently active timer, or N to focus the "Add Timer" input field would be a great little UX enhancement.
Great job getting this out there!