validation additions for not existing user in signup

add missing to last merge

changed milestone to %Meilenstein 2: Erweiterte Funktionalität und verbesserte Benutzererfahrung

requested review from @s86116

assigned to @s86116

enabled an automatic merge when the pipeline for 3580bb38 succeeds

aborted the automatic merge because source branch was updated

added 2 commits

• 5a1c2340 - small auth additions and some nav fixes
• aa8f46df - header done

Compare with previous version

mentioned in commit 7d188f3a

merged