I noticed that the favicon of http://127.0.0.1:8384 is a .png. I noticed it when I installed it as a PWA and saw that the desktop shortcut it makes makes a low quality icon. I notice now that the Syncthing icon at the top-left of this forum is also .png with the same issue when zooming in. But the exact same logo at the top-left of http://127.0.0.1:8384 is a .svg, so I suggest to follow its example.