📊 Calculate and show completion percentages per region

Implements #4 (closed), see https://gitlab.com/manuelgrabowski/team-tracking/-/issues/4#note_1378778144 for showcase screenshot. Our issues are currently also updated with the code in this MR.

Code is getting a little ugly now (looking at new line 93), but I think meaningfully refactoring that is a bigger effort that we already agreed on not including in 🍐🚀🦊💜v0 (and possibly ever for a project like this).