add header with language selector and theme toggle

[JeremyDev87]

Summary

Implements the responsive header navigation system for the landing page, addressing all requirements from #319.

• Header: Sticky navigation bar with backdrop blur, logo/brand link, desktop nav links, and mobile menu
• ThemeToggle: Light/dark/system mode switching powered by next-themes with dropdown menu
• LanguageSelector: 5-language locale switcher (en, ko, ja, zh-CN, es) using next-intl navigation
• MobileMenu: Sheet-based overlay containing nav links, theme toggle, and language selector
• i18n: Header translation messages added for all 5 supported locales
• UI: Added Sheet component from shadcn/ui (radix-ui) and radix-ui dependency

Files Changed

Area	Files
Components	components/Header/index.tsx, LanguageSelector.tsx, MobileMenu.tsx, ThemeToggle.tsx, constants.ts
UI	components/ui/sheet.tsx
i18n	i18n/navigation.ts, messages/{en,ko,ja,zh-CN,es}.json
Layout	src/app/[locale]/layout.tsx
Tests	__tests__/components/Header/{Header,LanguageSelector,MobileMenu,ThemeToggle}.test.tsx
Test Helpers	__tests__/__helpers__/{navigation-mock,next-themes-mock,next-intl-mock}.ts
Dependencies	package.json, yarn.lock (radix-ui added)

Test Plan

• Header renders on all locale pages (/en, /ko, /ja, /zh-CN, /es)
• Theme toggle switches between light, dark, and system modes
• Language selector navigates to correct locale route
• Mobile menu opens/closes with sheet overlay
• Mobile menu contains nav links, theme, and language controls
• Header is sticky with backdrop blur on scroll
• Responsive layout: desktop nav hidden on mobile, hamburger visible
• Keyboard navigation and accessibility (ARIA labels)
• All unit tests pass (yarn workspace codingbuddy-landing-page test)

Closes #319