fix: normalize email during user signup

[ThewindMom]

Summary

• Define the missing normalizeEmail helper used by usersignup.
• Reuse the normalized email for signup lookup, username, email, and normalizedEmail so case/whitespace variants resolve consistently.

Why

apps/OpenSignServer/cloud/parsefunction/usersignup.js currently calls normalizeEmail(...) inside saveUser(), but the helper is not defined or imported. In the published server image this causes the signup cloud function to fail with a ReferenceError before a self-hosted tenant/user can be created.

Validation

• node --check apps/OpenSignServer/cloud/parsefunction/usersignup.js
• node -e 'import("./cloud/parsefunction/usersignup.js")' from apps/OpenSignServer after npm ci
• npm exec -- prettier --check apps/OpenSignServer/cloud/parsefunction/usersignup.js

Note: npm run lint currently fails before linting this change because the server package uses ESLint 9.39.4 with only .eslintrc.json; ESLint 9 requires eslint.config.*.

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