[ENHANCEMENT]: Rework AuthService

[cv5ch]

Description

The AuthService should be reworked as it's very messy and most likely contains not yet found bugs.

1. The main problem I see is storing the authenticated state in too many places which could easily get out of sync somewhere and lead to awkward bugs:

• user: BehaviorSubject<AuthData | null>

• _authUser$: BehaviorSubject<AuthUser | null>

• isAuthenticated: boolean

• logged: ReplaySubject

• userLoggedIn: Subject

2. Another problem may be using an EventEmitter in a service as it's intended for Components (see https://angular.dev/api/core/EventEmitter):

• authChanged: EventEmitter

3. JWT decoding: Buffer should be replaced with a browser-safe base64 decoder that properly handles URL-safe base64 padding.

4. Login Function:

• Credentials are sent both in header and body which is redundant and increases the exposure surface -> Decide for one method depending on what the server requires (I think the body variant)

• Username and password are base64 encoded with "window.btoa" which only supports LATIN1/ASCII and could lead to locking users out, if they change their password to containing an UTF8-string like for example the German ä, ö, ü or French é, è, î etc. see subissue [BUG]: Username/Password base64 encoding bug #1915