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/LoginPage.tsx

@@ -3,9 +3,11 @@ import React, { useState } from 'react';
 import { motion } from 'framer-motion';
 import { Lock, User } from 'lucide-react';
 import toast from 'react-hot-toast';
+import { useTranslation } from 'react-i18next';
 import { useAuth } from '../utils/AuthContext';
 
 const LoginPage: React.FC = () => {
+  const { t } = useTranslation();
   const { login } = useAuth();
   const [username, setUsername] = useState('');
   const [password, setPassword] = useState('');
@@ -14,14 +16,14 @@ const LoginPage: React.FC = () => {
   const handleSubmit = async (e: React.FormEvent) => {
     e.preventDefault();
     if (!username || !password) {
-      toast.error('Introduce usuario y contraseña');
+      toast.error(t('auth.credentialsRequired'));
       return;
     }
 
     setIsSubmitting(true);
     try {
-      await login(username, password); 
-      toast.success('Sesión iniciada');
+      await login(username, password);
+      toast.success(t('auth.sessionStarted'));
     } catch (err) {
       console.error('Error en login', err);
       const msg =
@@ -48,14 +50,14 @@ const LoginPage: React.FC = () => {
             Beyond Diagnostic
           </h1>
           <p className="text-sm text-slate-500">
-            Inicia sesión para acceder al análisis
+            {t('auth.loginTitle')}
           </p>
         </div>
 
         <form onSubmit={handleSubmit} className="space-y-4">
           <div className="space-y-1">
             <label className="block text-sm font-medium text-slate-700">
-              Usuario
+              {t('auth.username')}
             </label>
             <div className="relative">
               <span className="absolute inset-y-0 left-0 pl-3 flex items-center text-slate-400">
@@ -73,7 +75,7 @@ const LoginPage: React.FC = () => {
 
           <div className="space-y-1">
             <label className="block text-sm font-medium text-slate-700">
-              Contraseña
+              {t('auth.password')}
             </label>
             <div className="relative">
               <span className="absolute inset-y-0 left-0 pl-3 flex items-center text-slate-400">
@@ -94,11 +96,11 @@ const LoginPage: React.FC = () => {
             disabled={isSubmitting}
             className="w-full inline-flex items-center justify-center rounded-2xl bg-indigo-600 text-white text-sm font-medium py-2.5 shadow-sm hover:bg-indigo-700 focus:outline-none focus:ring-2 focus:ring-offset-2 focus:ring-indigo-500 disabled:opacity-60 disabled:cursor-not-allowed"
           >
-            {isSubmitting ? 'Entrando…' : 'Entrar'}
+            {isSubmitting ? t('auth.enteringButton') : t('auth.enterButton')}
           </button>
 
           <p className="text-[11px] text-slate-400 text-center mt-2">
-            La sesión permanecerá activa durante 1 hora.
+            {t('auth.sessionInfo')}
           </p>
         </form>
       </motion.div>