[AI Config] Simple-mode config contents

[ndoschek]

PO: @ndoschek

The Simple / Pro toggle and Simple-mode container shipped in #315 as a placeholder. The actual Simple-mode page contents are the subject of the onboarding concept in #278. This ticket delivers those contents into the AI Configuration view's Simple-mode container.

Acceptance scenario

Scenario: Set up AI with minimal effort in Simple mode
  Given a user opens the AI Configuration view in Simple mode
  When the user follows the Simple-mode page
  Then the user is guided through the minimum setup needed to use AI in Theia
  And advanced options are not surfaced unless the user switches to Pro mode

Scope

• Follow the onboarding concept's design for the Simple-mode page.
• Implement the page inside the Simple-mode container from [AI Config] Simple / Pro toggle and placeholder #315
• Remove any welcome-screen actions that become obsolete with this page now.

Out of scope

• The requirements of the Simple-mode page. Defined by the onboarding concept in Theia AI Onboarding Experience #278