refactor: Add events package

[Planlos5000]

Description

This MR will refactor the event handling and introduces a new package for handling the events. This MR will also fix the bug that is explained in #6186 and #6197.
Because of cyclic imports this MR also restructures the INIT event which then does not send the whole board. The frontend now has to fetch the board.

Changelog

• introduce events package
• add tests for events package
• refactor INIT event

Checklist

• I have performed a self-review of my own code
• I have written and understand every part of this contribution myself - if AI tools were used, I have thoroughly reviewed and verified all changes
• I have commented my code, particularly in hard-to-understand areas
• The light- and dark-theme are both supported and tested
• The design was implemented and is responsive for all devices and screen sizes
• The application was tested in the most commonly used browsers (e.g. Chrome, Firefox, Safari)

(Optional) Visual Changes