feat(i18n): Implement multi-language support (en, zh-CN, zh-TW, ja) with auto-detection

[yozurukt]

Close #49
This PR adds internationalization (i18n) support to Moodist using i18next and react-i18next, with the UI now available in English, Simplified Chinese, Traditional Chinese, and Japanese.

Key Changes:

• Multi-language UI: Extracted all visible UI text and accessibility (a11y) strings into separate locale files (en, zh-CN, zh-TW, ja) for easy translation and maintenance.
• Automatic Language Redirect: Implemented a client-side redirect on / that detects the user's preferred language via navigator.languages, and routes them to the appropriate localized version (uses sessionStorage to prevent redirect loops).
• CJK Font Support (LXGW WenKai Lite): Integrated LXGW WenKai Lite and applied it conditionally for zh-CN, zh-TW, and ja locales using @font-face and :lang() selectors to enhance readability for CJK characters.
• Font Size Adjustment for CJK: Slightly increased the base font size for CJK locales via CSS variables for improved visual balance and legibility.
• Accessibility Improvements: All accessibility strings (e.g., aria-label, alt, etc.) have been internationalized and moved into locale files for consistency.
• Stylelint Config Fix: Updated Stylelint settings to allow Astro’s :global() pseudo-class, resolving an issue when styling set:html content in the footer.

Note: I'm still new to front-end development, so I’d really appreciate any feedback on the implementation, especially on best practices or things I might have overlooked!