Ensure email is lowercase and check availability on user creation

[ChinoKou]

This pull request adds email normalization and uniqueness validation to the user creation logic in backend/internal/user.js. Now, emails are converted to lowercase and trimmed, and the system checks if the email is already in use before proceeding.

User validation improvements:

• Emails are now normalized (lowercased and trimmed) before processing to ensure consistency.
• Added a uniqueness check for emails during user creation, throwing a validation error if the email is already in use.

[ChinoKou]

Fixes: #4983 #5323

[nginxproxymanagerci]

Docker Image for build 1 is available on DockerHub:

nginxproxymanager/nginx-proxy-manager-dev:pr-5442

Note

Ensure you backup your NPM instance before testing this image! Especially if there are database changes.
This is a different docker image namespace than the official image.

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