UI Polish for Self-hosted models

What does this MR do and why?

• Migrating the heading area to the page-heading component, which changes the heading from h2 to h1.
• Updates specs.
• Removes bottom border from tabs.
• Moves the main cta Add self-hosted model to the page heading to make it more consistent.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Prequisites

You will need an active GitLab ultimate license.

1. Start GDK instance in self-managed mode:

GITLAB_SIMULATE_SAAS=0 && gdk start

2. In the rails console, enable Code Suggestions and the custom models experiment:

Feature.enable(:ai_custom_model)
Feature.enable(:custom_models_vue_app)
Feature.enable(:custom_models_feature_settings_vue_app)
ApplicationSetting.first.update(duo_features_enabled: true)

Test it!

1. Log in as an admin
2. Visit the page Admin Area > Self-hosted models