Fix login validation messages not showing on submit

[calm329]

Prerequisites

• I have added steps to test this contribution in the description below

This fixes #21300

Description

Problem:
When clicking the Login button without entering credentials, validation messages were not displayed. Users only saw validation errors after focusing on the input fields first.

Solution:
Added oninvalid event handlers to both username and password inputs in auth.element.ts. The invalid event fires automatically when checkValidity() is called on a form with invalid inputs, triggering the existing validateInput() function to display error messages.

Changes:

• src/Umbraco.Web.UI.Login/src/auth.element.ts: Added input.oninvalid handlers in createFormLayoutItem() and createFormLayoutPasswordItem()

How to test:

1. Navigate to the Umbraco login page
2. Without entering any credentials, click the "Login" button
3. ✅ Validation messages should appear for both username and password fields
4. Enter only username, click "Login" → validation message shows for password
5. Enter only password, click "Login" → validation message shows for username

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Contribution by Gittensor, see my contribution statistics at https://gittensor.io/miners/details?githubId=148254234

[github-actions]

Hi there @calm329, thank you for this contribution! 👍

While we wait for one of the Core Collaborators team to have a look at your work, we wanted to let you know about that we have a checklist for some of the things we will consider during review:

• It's clear what problem this is solving, there's a connected issue or a description of what the changes do and how to test them
• The automated tests all pass (see "Checks" tab on this PR)
• The level of security for this contribution is the same or improved
• The level of performance for this contribution is the same or improved
• Avoids creating breaking changes; note that behavioral changes might also be perceived as breaking
• If this is a new feature, Umbraco HQ provided guidance on the implementation beforehand
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[andr317c]

LGTM! Thanks for contributing 😄

[calm329]

Thanks for your review & merge! I'd love to contribute more if possible!

[andr317c]

Cherry picked for 17.1 🎉