add CodeExample widget with tabs and syntax highlighting

[JeremyDev87]

Summary

Implements the CodeExample widget for the landing page, providing before/after code comparison to showcase the value of Codingbuddy.

• Add CodeExample widget with tabbed before/after code comparison using shadcn/ui Tabs
• Integrate syntax highlighting via prism-react-renderer
• Add copy-to-clipboard functionality with success/failure feedback via CopyButton
• Add i18n translations for all 5 languages (en, ko, ja, zh-CN, es)
• Improve Tabs component accessibility (motion-reduce:transition-none, focus-visible ring on TabsContent)
• Simplify CodeExampleProps type by removing beforeCode/afterCode props (data is now self-contained)
• Restructure tests into dedicated CodeExample/ directory with per-component test files

Changes

New Files

• widgets/CodeExample/data/examples.ts - Before/after code example data
• widgets/CodeExample/lib/copyToClipboard.ts - Clipboard utility
• widgets/CodeExample/ui/CodeBlock.tsx - Syntax-highlighted code block component
• widgets/CodeExample/ui/CopyButton.tsx - Copy button with feedback states
• __tests__/widgets/CodeExample/CodeExample.test.tsx - Widget integration tests
• __tests__/widgets/CodeExample/CodeBlock.test.tsx - CodeBlock unit tests
• __tests__/widgets/CodeExample/CopyButton.test.tsx - CopyButton unit tests
• __tests__/widgets/CodeExample/copyToClipboard.test.ts - Clipboard utility tests

Modified Files

• widgets/CodeExample/index.tsx - Full rewrite with tabs, syntax highlighting, i18n
• components/ui/tabs.tsx - Accessibility improvements
• types/widgets.ts - Simplified CodeExampleProps to WidgetProps alias
• src/app/[locale]/@code_example/page.tsx - Simplified slot (no more code props)
• messages/{en,ko,ja,zh-CN,es}.json - Updated codeExample i18n keys
• package.json - Added prism-react-renderer dependency
• __tests__/__helpers__/next-intl-mock.ts - Namespace-aware mock for multiple widgets

Test Plan

• Verify tab switching between Before/After views
• Verify syntax highlighting renders correctly
• Verify copy-to-clipboard works and shows feedback
• Verify responsive layout on mobile
• Verify i18n translations for all 5 locales
• Run yarn workspace landing-page test - all tests pass
• Run yarn workspace landing-page build - build succeeds

Closes #316