Refresh early guide screenshots and remove Theme Basics

[shai-almog]

Summary:

• Remove the obsolete Theme Basics designer-tool chapter and its chapter-only images.
• Add demo-backed generators for 27 applicable screenshots before Advanced Theming and commit the regenerated PNGs.
• Add a developer-guide workflow gate that regenerates those screenshots and compares them to the committed files.

Local validation:

• python3 scripts/developer-guide/validate-guide-snippets.py
• python3 scripts/developer-guide/find_unused_images.py docs/developer-guide --allow-unused img/autocomplete.png --allow-unused img/native-theme-menu.png --allow-unused img/tabs.png
• asciidoctor --require rouge --failure-level WARN --trace -o /tmp/developer-guide-pre-ch5.html docs/developer-guide/developer-guide.asciidoc
• mvn -B -ntp -Dgenerate-gui-sources-done=true -Dcodename1.platform=javase -f docs/demos/pom.xml process-classes
• scripts/developer-guide/compile-android-demo-sources.sh

Note: the screenshot profile is wired for Ubuntu/xvfb in CI. The local macOS forked AWT path aborts before screenshot generation, so I am using the PR job as the authoritative screenshot execution check and will fix any CI failure.

[github-actions]

Cloudflare Preview

• URL: https://pr-5342-website-preview.codenameone.pages.dev
• Branch: pr-5342-website-preview

[github-actions]

Developer Guide build artifacts are available for download from this workflow run:

• Developer Guide HTML package
• Developer Guide PDF
• AsciiDoc linter report
• Vale report
• Unused image report
• Paragraph capitalization report
• LanguageTool report

Developer Guide quality checks:

• AsciiDoc linter: No issues found (report)
• Vale: No alerts found (report)
• Paragraph capitalization: No paragraph capitalization issues (report)
• LanguageTool: No grammar matches (report)
• Image references: No unused images detected (report)