Add Spanish translation support to OpenTeraPlus

[Copilot]

This PR adds comprehensive Spanish language support to OpenTeraPlus using Qt linguist format, enabling the application to be fully localized for Spanish-speaking users.

Changes Made

🆕 New Translation File

• client/resources/translations/openteraplus_es.ts - Complete Spanish translation template with 1,226 translatable strings
• Uses proper Qt linguist format with language code es_ES
• All strings marked as type="unfinished" ready for professional translation

🔧 Build System Integration

• client/resources/CMakeLists.txt - Added Spanish translation to the translations list
• client/src/CMakeLists.txt - Integrated Spanish translation into Qt linguist build pipeline (qt_add_lupdate/qt_add_lrelease)

💻 Application Code Updates

• client/src/ClientApp.cpp - Added "es" to supported languages and Spanish locale handling (QLocale::Spanish)
• client/src/dialogs/EmailInviteDialog.cpp - Added Spanish ("Español") option to language selection dropdown

Technical Implementation

✅ Qt Linguist Best Practices: Follows standard Qt translation workflow
✅ Minimal Changes: Only 7 lines of application code modified
✅ Build Integration: Automatic .qm file generation from .ts source
✅ Backward Compatible: No existing functionality affected

Usage for Translators

1. Open client/resources/translations/openteraplus_es.ts in Qt Linguist
2. Complete the Spanish translations for all 1,226 strings
3. Build system will automatically generate .qm files
4. Users can select Spanish from application preferences

User Experience

Once translations are completed:

• Full Spanish UI localization
• Spanish email invitation templates
• Seamless language switching
• Native Spanish locale formatting

The implementation is ready for professional Spanish translators to complete the localization work.

Fixes #105.

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