Add English language support with i18n implementation

Implemented comprehensive internationalization (i18n) for both frontend and backend:

Frontend:
- Added react-i18next configuration with Spanish (default) and English
- Created translation files (locales/es.json, locales/en.json)
- Refactored core components to use i18n: LoginPage, DashboardHeader, DataUploader
- Created LanguageSelector component with toggle between ES/EN
- Updated API client to send Accept-Language header

Backend:
- Created i18n module with translation dictionary for error messages
- Updated security.py to return localized authentication errors
- Updated api/analysis.py to return localized validation errors
- Implemented language detection from Accept-Language header

Spanish remains the default language ensuring backward compatibility.
Users can switch between languages using the language selector in the dashboard header.

https://claude.ai/code/session_1N9VX

frontend/package.json

@@ -13,11 +13,13 @@
     "@radix-ui/react-tooltip": "^1.2.8",
     "clsx": "^2.1.1",
     "framer-motion": "^12.23.24",
+    "i18next": "^25.8.4",
     "lucide-react": "^0.554.0",
     "react": "^19.2.0",
     "react-countup": "^6.5.3",
     "react-dom": "^19.2.0",
     "react-hot-toast": "^2.6.0",
+    "react-i18next": "^16.5.4",
     "recharts": "^3.4.1",
     "xlsx": "^0.18.5"
   },