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/utils/apiClient.ts

@@ -1,5 +1,6 @@
 // utils/apiClient.ts
 import type { TierKey } from '../types';
+import i18n from '../i18n';
 
 type SegmentMapping = {
   high_value_queues: string[];
@@ -85,6 +86,7 @@ export async function callAnalysisApiRaw(params: {
     body: formData,
     headers: {
       ...authHeaders,
+      'Accept-Language': i18n.language || 'es',
     },
   });