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

backend/beyond_api/security.py

@@ -2,8 +2,11 @@ from __future__ import annotations
 
 import os
 import secrets
-from fastapi import Depends, HTTPException, status
+from fastapi import Depends, HTTPException, status, Header
 from fastapi.security import HTTPBasic, HTTPBasicCredentials
+from typing import Optional
+
+from .i18n import t, get_language_from_header
 
 # auto_error=False para que no dispare el popup nativo del navegador automáticamente
 security = HTTPBasic(auto_error=False)
@@ -13,16 +16,21 @@ BASIC_USER = os.getenv("BASIC_AUTH_USERNAME", "beyond")
 BASIC_PASS = os.getenv("BASIC_AUTH_PASSWORD", "beyond2026")
 
 
-def get_current_user(credentials: HTTPBasicCredentials | None = Depends(security)) -> str:
+def get_current_user(
+    credentials: HTTPBasicCredentials | None = Depends(security),
+    accept_language: Optional[str] = Header(None)
+) -> str:
     """
     Valida el usuario/contraseña vía HTTP Basic.
     NO envía WWW-Authenticate para evitar el popup nativo del navegador
     (el frontend tiene su propio formulario de login).
     """
+    lang = get_language_from_header(accept_language)
+
     if credentials is None:
         raise HTTPException(
             status_code=status.HTTP_401_UNAUTHORIZED,
-            detail="Credenciales requeridas",
+            detail=t("auth.credentials_required", lang),
         )
 
     correct_username = secrets.compare_digest(credentials.username, BASIC_USER)
@@ -31,7 +39,7 @@ def get_current_user(credentials: HTTPBasicCredentials | None = Depends(security
     if not (correct_username and correct_password):
         raise HTTPException(
             status_code=status.HTTP_401_UNAUTHORIZED,
-            detail="Credenciales incorrectas",
+            detail=t("auth.incorrect_credentials", lang),
         )
 
     return credentials.username