feat: modernise auth layout and views to align with admin template

[juzaweb]

This PR updates the authentication layout and views to match the look and feel of the modern admin template.

Specifically:

• Brought over dns-prefetch links and the flag-icons stylesheet from the core admin layout into the auth layout.
• Fixed a syntax issue in auth.blade.php where an @if conditional was incorrectly closed with an @endguest.
• Refactored login, register, forgot-password, reset-password, and verification-notice views to use the <div class="card card-outline card-primary"> layout for a consistent and modern UI.
• Corrected a CDN typo: dnjs to cdnjs.

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PR created automatically by Jules for task 10770943318239407803 started by @juzaweb

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