fix(core): Do not validate email when LDAP is enabled

[RicardoE105]

Understanding the Issue

The core problem stems from a previous PR (#12375) where we started validating the email via the DTO. This created an unexpected issue for LDAP users because the "email" field might not actually contain an email address but rather a username based on LDAP configuration. The validation was rejecting non-email formats, preventing legitimate LDAP users from logging in.

Solution Approach

1. Renaming the field from "email" to "emailOrLdapUsername" to better represent its purpose
2. Using the updated LoginRequestDto in the frontend
3. Only validating as an email when LDAP is disabled
4. Adding tests to prevent regression

I note that a better long-term solution would be to separate authentication flows between administrators and LDAP members, but this larger refactoring is out of scope for this immediate fix.

Affected Users

This affects instances on [email protected] and later where LDAP is configured to use a non-email identifier (anything other than "mail").

Related Linear tickets, Github issues, and Community forum posts

https://linear.app/n8n/issue/CAT-672/community-issue-ldap-users-wrong-email-validation
#13462

Review / Merge checklist

• PR title and summary are descriptive. ([conventions](../blob/master/.github/pull_request_title_conventions.md))
• [Docs updated](https://github.com/n8n-io/n8n-docs) or follow-up ticket created.
• Tests included.
• PR Labeled with release/backport (if the PR is an urgent fix that needs to be backported)

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ckages/frontend/editor-ui/src/views/SigninView.vue	77.77%	2 Missing ⚠️
packages/frontend/editor-ui/src/api/users.ts	0.00%	1 Missing ⚠️

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