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/api/analysis.py

@@ -7,11 +7,12 @@ import math
 from uuid import uuid4
 from typing import Optional, Any, Literal
 
-from fastapi import APIRouter, UploadFile, File, Form, HTTPException, Depends
+from fastapi import APIRouter, UploadFile, File, Form, HTTPException, Depends, Header
 from fastapi.responses import JSONResponse
 
 from beyond_api.security import get_current_user
 from beyond_api.services.analysis_service import run_analysis_collect_json
+from beyond_api.i18n import t, get_language_from_header
 
 # Cache paths - same as in cache.py
 CACHE_DIR = Path(os.getenv("CACHE_DIR", "/data/cache"))
@@ -52,6 +53,7 @@ async def analysis_endpoint(
     economy_json: Optional[str] = Form(default=None),
     analysis: Literal["basic", "premium"] = Form(default="premium"),
     current_user: str = Depends(get_current_user),
+    accept_language: Optional[str] = Header(None),
 ):
     """
     Ejecuta el pipeline sobre un CSV subido (multipart/form-data) y devuelve
@@ -62,12 +64,13 @@ async def analysis_endpoint(
     - "premium": usa la configuración completa por defecto
                  (p.ej. beyond_metrics_config.json), sin romper lo existente.
     """
+    lang = get_language_from_header(accept_language)
 
     # Validar `analysis` (por si llega algo raro)
     if analysis not in {"basic", "premium"}:
         raise HTTPException(
             status_code=400,
-            detail="analysis debe ser 'basic' o 'premium'.",
+            detail=t("analysis.invalid_type", lang),
         )
 
     # 1) Parseo de economía (si viene)
@@ -78,7 +81,7 @@ async def analysis_endpoint(
         except json.JSONDecodeError:
             raise HTTPException(
                 status_code=400,
-                detail="economy_json no es un JSON válido.",
+                detail=t("analysis.invalid_economy_json", lang),
             )
 
     # 2) Guardar el CSV subido en una carpeta de trabajo
@@ -159,23 +162,26 @@ async def analysis_cached_endpoint(
     economy_json: Optional[str] = Form(default=None),
     analysis: Literal["basic", "premium"] = Form(default="premium"),
     current_user: str = Depends(get_current_user),
+    accept_language: Optional[str] = Header(None),
 ):
     """
     Ejecuta el pipeline sobre el archivo CSV cacheado en el servidor.
     Útil para re-analizar sin tener que subir el archivo de nuevo.
     """
+    lang = get_language_from_header(accept_language)
+
     # Validar que existe el archivo cacheado
     if not CACHED_FILE.exists():
         raise HTTPException(
             status_code=404,
-            detail="No hay archivo cacheado en el servidor. Sube un archivo primero.",
+            detail=t("analysis.no_cached_file", lang),
         )
 
     # Validar `analysis`
     if analysis not in {"basic", "premium"}:
         raise HTTPException(
             status_code=400,
-            detail="analysis debe ser 'basic' o 'premium'.",
+            detail=t("analysis.invalid_type", lang),
         )
 
     # Parseo de economía (si viene)
@@ -186,7 +192,7 @@ async def analysis_cached_endpoint(
         except json.JSONDecodeError:
             raise HTTPException(
                 status_code=400,
-                detail="economy_json no es un JSON válido.",
+                detail=t("analysis.invalid_economy_json", lang),
             )
 
     # Extraer metadatos del CSV
@@ -204,7 +210,7 @@ async def analysis_cached_endpoint(
     except Exception as e:
         raise HTTPException(
             status_code=500,
-            detail=f"Error ejecutando análisis: {str(e)}",
+            detail=t("analysis.execution_error", lang, error=str(e)),
         )
 
     # Limpiar NaN/inf para que el JSON sea válido