Add comprehensive internationalization (i18n) with Spanish language support

This major feature introduces complete internationalization architecture with Spanish (Mexico) as the first additional language:

## New Features:
- **I18n Architecture**: Complete client-side and server-side internationalization system
- **Spanish (Mexico) Support**: Full es-MX translations for all user-facing text
- **Language Selector**: Dynamic language switching UI component in header
- **API Endpoints**: RESTful endpoints for serving translations (/api/i18n)
- **Progressive Enhancement**: Language detection via Accept-Language header and cookies

## Technical Implementation:
- **Frontend**: Client-side i18n.js with automatic DOM translation and language selector
- **Backend**: Server-side i18n service with locale detection middleware
- **Build Process**: Automated copying of translation files to dist/ directory
- **Responsive Design**: Language selector integrated into header controls layout

## Files Added:
- public/i18n.js - Client-side internationalization library
- src/i18n/index.ts - Server-side i18n service
- src/i18n/locales/en.json - English translations
- src/i18n/locales/es-MX.json - Spanish (Mexico) translations
- src/routes/i18n.ts - API endpoints for translations

## Files Modified:
- package.json - Updated build process to include i18n files
- public/index.html - Added i18n attributes and language selector
- public/app.js - Integrated dynamic translation updates
- src/server.ts - Added locale detection middleware
- src/scss/pages/_index.scss - Language selector styling

This implementation supports easy addition of future languages and maintains backward compatibility while providing a seamless multilingual experience.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -13,8 +13,9 @@
     "watch-css": "sass src/scss/main.scss public/style.css --watch --style=expanded --source-map",
     "dev-with-css": "concurrently \"npm run watch-css\" \"npm run dev\"",
     "dev-with-css:ts": "concurrently \"npm run watch-css\" \"npm run dev:ts\"",
-    "build": "npm run build:ts && npm run build-css",
+    "build": "npm run build:ts && npm run build-css && npm run copy-i18n",
     "build:ts": "tsc",
+    "copy-i18n": "mkdir -p dist/i18n/locales && cp -r src/i18n/locales/* dist/i18n/locales/",
     "test": "jest --runInBand --forceExit",
     "test:coverage": "jest --coverage",
     "lint": "eslint src/ tests/",