fix: change http response of passphrase request

[mateo-ivc]

Description

Change of http status codes when sending a join request with an incorrect passphrase to a protected board for better differentiation during error evaluation.

Changelog

In 'api/boards.go': add check for empty passphrase and change of status code from ForbiddenRequest to BadRequest In 'api/participant.ts': Added status codes to the return value when a 400 request is received when trying to join a board

Checklist

• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] The light- and dark-theme are both supported and tested
• [x] The design was implemented and is responsive for all devices and screen sizes
• [x] The application was tested in the most commonly used browsers (e.g. Chrome, Firefox, Safari)

(Optional) Visual Changes