#32111987 Themes: Add AI Builder banner component

Fixes DOTTHEM-307

Proposed Changes

Desktop	Mobile

• Add a new AIBuilderBanner component to promote the AI website builder from the themes showcase.
• The banner includes a title, description, CTA button linking to /setup/ai-site-builder, a decorative background illustration, and a WordPress + Big Sky byline.
• Click tracking via calypso_themeshowcase_ai_builder_banner_click.

Note: the banner is NOT integrated with the showcase just yet. Size and margins are temporary and depend on how it will be inserted into the grid.

Why are these changes being made?

This banner will be placed in the themes showcase to surface the AI website builder as an alternative to browsing themes manually.

Testing Instructions

• Apply the following uncommitted change to client/my-sites/themes/theme-showcase.jsx to render the banner in the showcase:
  • Import AIBuilderBanner from calypso/my-sites/themes/banners-modern/ai-builder-banner
  • Add { this.isThemeShowcaseModern() && <AIBuilderBanner /> } as a child of <ThemesSelection> in the allThemes method
• Navigate to /themes (logged out) to see the banner at the second row of the theme grid.
• Verify the banner renders correctly at various viewport widths.
• Click “Start with AI” and verify it navigates to /setup/ai-site-builder.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple, Atomic, and self-hosted Jetpack sites?
• Have you checked for TypeScript, React or other console errors?
• Have you tested accessibility for your changes?
• Have you used memoizing on expensive computations?
• Have we added the “[Status] String Freeze” label as soon as any new strings were ready for translation?
  • For UI changes, have we tested the change in various languages?
• For changes affecting Jetpack: Have we added the “[Status] Needs Privacy Updates” label?