Missing shortened months in german (maybe in other languages too) and weekview not working

[JacobBraun]

It seems like month abbreviations such as "March" (German: März) are not available in the const SHORTENED list. The issue might be due to using "März" as the key, which can't be resolved. It could be helpful to make these names configurable in the Gantt constructor.

Additionally, when I add a task starting on 2024-05-01, it appears to begin on 2024-04-28 or 29. The "today" marker is also off by a few days.