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/components/LanguageSelector.tsx

@@ -0,0 +1,29 @@
+import React from 'react';
+import { useTranslation } from 'react-i18next';
+import { Languages } from 'lucide-react';
+
+export function LanguageSelector() {
+  const { i18n } = useTranslation();
+
+  const toggleLanguage = () => {
+    const newLang = i18n.language === 'es' ? 'en' : 'es';
+    i18n.changeLanguage(newLang);
+    localStorage.setItem('language', newLang);
+  };
+
+  const currentLang = i18n.language === 'es' ? 'ES' : 'EN';
+  const nextLang = i18n.language === 'es' ? 'EN' : 'ES';
+
+  return (
+    <button
+      onClick={toggleLanguage}
+      className="inline-flex items-center gap-1.5 px-3 py-1.5 bg-slate-100 text-slate-700 rounded-full text-xs font-medium hover:bg-slate-200 transition-colors cursor-pointer"
+      title={`Switch to ${nextLang}`}
+    >
+      <Languages className="w-3.5 h-3.5" />
+      <span className="font-semibold">{currentLang}</span>
+    </button>
+  );
+}
+
+export default LanguageSelector;