feat(i18n): Add English language support with vue-i18n

[Eeman1113]

Summary

• Add comprehensive internationalization support using vue-i18n
• Create complete English and Chinese locale files
• Update all major UI components to use translation keys
• Keep Chinese as default language - non-breaking change
• Add language switcher in navbar for easy language toggling
• Language preference saved to localStorage

Changes Made

• Added vue-i18n@9 dependency
• Created English locale file (frontend/src/locales/en.js) with complete translations
• Created Chinese locale file (frontend/src/locales/zh.js) with existing Chinese text
• Updated frontend/src/main.js to initialize i18n instance
• Created LanguageSwitcher component for language toggling
• Refactored components to use $t() translation function:
  • Home.vue: Complete UI translation including hero section, workflow steps, console
  • MainView.vue: Navigation and step names translation
  • HistoryDatabase.vue: History interface translation
• Fixed template structure issues in Home.vue
• Added language switcher to navbar in Home.vue

Benefits

• Makes MiroFish accessible to international English-speaking users
• Provides foundation for additional language support
• Non-breaking change - Chinese remains default for existing users
• Easy language switching between Chinese and English
• Language preference persists via localStorage
• Clean separation of UI text from component logic
• Easy to extend with additional languages

Testing

• Tested with both English and Chinese locales
• All major UI components properly translate
• Language switcher works correctly
• Language preference saved and restored
• No breaking changes to existing Chinese functionality
• Default language set to Chinese for backward compatibility

Notes

• Chinese (zh) is set as default language - existing users see no change
• Language switcher added to navbar for easy language toggling
• Users can easily switch between Chinese and English
• All existing Chinese text preserved and properly organized in locale files
• This is a non-breaking change - existing users continue to see Chinese UI

[666ghj]

The project already has i18n support in place — please refer to [PR #428] for reference.