feat(i18n): add client-side language switching, translations, and html-lang synchronization

[cto-new]

Summary

Add client-side i18n support with zh-CN and en-US, including a persistent language store, translation hooks, language switchers, and html lang synchronization. Extend hero placeholders to support mobile/desktop variants.

Details

• Implement a client-side i18n store and translation hooks with zh-CN/en-US dictionaries
• Add translations for core texts and hero section; split placeholders into mobile/desktop variants
• Introduce LanguageHtmlAttribute to align document.html.lang with the selected language and apply in root layout
• Add language switcher UI components (LanguageSwitcher and SimpleLanguageSwitcher) for full and compact modes
• Update app layout to initialize default HTML lang and wire in new i18n logic
• Persist language preference in localStorage and apply changes to UI immediately
• Note: No server-side API changes; this is a client-side enhancement

Warning: Task VM test is not passing, cto.new will perform much better if you fix the setup