configure next-intl i18n for 5 languages

[JeremyDev87]

Summary

• Configure next-intl v4 with App Router pattern for internationalization support
• Add 5 language support: English, Korean, Chinese (Simplified), Japanese, Spanish
• Integrate i18n routing, middleware, message files, and NextIntlClientProvider into existing locale layout
• Refactor lib/locale.ts to derive from next-intl routing config as single source of truth
• Tighten CSP script-src policy for production (remove unsafe-eval/unsafe-inline)

Architecture

middleware.ts (locale detection/redirect)
    ↓
i18n/routing.ts (defineRouting - source of truth)
    ↓                    ↓
lib/locale.ts         i18n/request.ts (getRequestConfig)
(re-export)               ↓
                    messages/{locale}.json
                         ↓
              [locale]/layout.tsx
              (NextIntlClientProvider)
                    ↓
              widgets/components (useTranslations)

Changes

New files

File	Description
i18n/routing.ts	Routing config with 5 locales, always prefix, en default
i18n/request.ts	Server request config with hasLocale validation and fallback
middleware.ts	next-intl middleware for locale detection and routing
messages/{en,ko,zh-CN,ja,es}.json	Translation files for hero, agents, codeExample, quickStart, faq sections
__tests__/i18n/routing.test.ts	Routing config tests
__tests__/i18n/messages.test.ts	Message key structure consistency tests
__tests__/i18n/request.test.ts	Request config logic + path traversal security tests
__tests__/i18n/layout.test.ts	generateStaticParams tests

Modified files

File	Description
package.json	Add next-intl@^4.8.2 dependency
next.config.ts	Wrap with createNextIntlPlugin, tighten CSP for production
lib/locale.ts	Derive SUPPORTED_LOCALES and DEFAULT_LOCALE from routing
__tests__/lib/locale.test.ts	Update tests for 5 locales, add case-sensitivity checks
src/app/[locale]/layout.tsx	Wrap with NextIntlClientProvider, call getMessages()

Test plan

• yarn workspace landing-page vitest run — all i18n and locale tests pass
• yarn workspace landing-page typecheck — no type errors
• Verify all 5 message files have identical key structure (covered by messages.test.ts)
• Verify hasLocale rejects path traversal attempts (covered by request.test.ts)
• Verify generateStaticParams produces entries for all 5 locales (covered by layout.test.ts)
• Verify middleware routes / to /en and handles all locale prefixes

Closes #313