implement widget-slot architecture with parallel routes

[JeremyDev87]

Summary

Implements the Widget-Slot Architecture (WSA) for the landing page using Next.js Parallel Routes, enabling independent loading states and error boundaries per widget slot.

• Add locale utility with DEFAULT_LOCALE, SUPPORTED_LOCALES, and validation helper
• Add shared components: SlotError (error boundary UI), Skeleton (loading primitive), SetDocumentLang (client-side locale)
• Add widget placeholders: AgentsShowcase, CodeExample, QuickStart with accessible markup (aria-labelledby, lang, data-testid)
• Add [locale] route group with 3 parallel route slots (@agents, @code_example, @quick_start), each with page, loading, error, and default files
• Update root page to redirect to default locale
• Add SlotProps type and refine WidgetProps.locale to SupportedLocale
• Add prefers-reduced-motion accessibility styles
• Update accessibility tests for the new architecture (widget sections, loading states, error states)

Commits

1. docs(landing-page): add widget-slot architecture implementation plan
2. feat(landing-page): add locale utility with supported locale validation
3. feat(landing-page): add shared components for widget slot architecture
4. feat(landing-page): add widget placeholder components with tests
5. feat(landing-page): implement parallel routes with locale slot layout

Test plan

• Verify locale.ts utility correctly validates supported locales (en, ko)
• Verify SlotError component renders with role="alert" and retry button
• Verify widget placeholders render with correct aria-labelledby, lang, and data-testid attributes
• Verify loading skeleton components have aria-busy="true"
• Verify parallel route slots render independently (each slot loads/errors without affecting others)
• Verify root / redirects to /en (default locale)
• Run yarn workspace landing-page test — all accessibility tests pass
• Verify axe-core reports zero violations for English and Korean locales

Closes #312