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/i18n.ts

@@ -0,0 +1,21 @@
+import i18n from 'i18next';
+import { initReactI18next } from 'react-i18next';
+import es from './locales/es.json';
+import en from './locales/en.json';
+
+// Configuración de i18next
+i18n
+  .use(initReactI18next)
+  .init({
+    resources: {
+      es: { translation: es },
+      en: { translation: en },
+    },
+    lng: localStorage.getItem('language') || 'es', // Español por defecto
+    fallbackLng: 'es', // Si falla una traducción, usa español
+    interpolation: {
+      escapeValue: false, // React ya escapa por defecto
+    },
+  });
+
+export default i18n;