Add internationalization support for Japanese and English

[msakai]

Summary

This PR adds comprehensive internationalization (i18n) support to the game, enabling it to display in both Japanese and English based on user language preferences or URL parameters.

Key Changes

• New i18n module (js/i18n.js): Created a centralized translation system with message dictionaries for Japanese and English, automatic language detection based on browser language preferences or ?lang= URL parameter, with Japanese as the fallback language
• Updated UI components: Modified title screen, game over screen, and stage completion screens to use the new t() translation function instead of hardcoded strings
• Manifest files: Added manifest.en.json for English localization and updated manifest.json to include language metadata
• HTML integration: Added script in index.html to set document language, update page title, and dynamically load the appropriate manifest file based on detected language
• Translated strings: All user-facing text now supports both languages including:
  • Game title and instructions
  • Score labels
  • Game state messages (GAME OVER, GAME CLEAR)

Implementation Details

• Language detection follows a priority order: URL parameter → browser language preferences → Japanese default
• The translation function gracefully falls back to Japanese if a key is missing in the current language
• Manifest switching is handled dynamically in the HTML to ensure PWA metadata matches the active language

https://claude.ai/code/session_01QAYkCU7d7Yjvu753eDBMiB