text: Title: Add a visual indication to Favicon/Tab
  Body: ![image](https://github.com/healthchecks/healthchecks/assets/12234510/6776c6bb-fae0-4f5c-9b7e-bfbab4ea3037)
  
  This is what we get when a monitor is down in a full tab. Howhever, if the tab is pinned on browser, we have no visual indication.
  
  [Uptime-Kuma](https://github.com/louislam/uptime-kuma) project has implemented this, for reference _(here both tabs pinned, side by side, where both have 2 monitors down)_:
  
  ![image](https://github.com/healthchecks/healthchecks/assets/12234510/26c6c97e-3091-4428-be45-cedc2278ff85)
  