refactor: implement i18n in core components (phase 1)

Refactored key components to use react-i18next translations:
- ErrorBoundary: error messages and labels
- ProgressStepper: step labels
- DimensionCard: health status labels, descriptions, benchmark text, action buttons
- DashboardTabs: back button, footer, default title

Added translation keys to es.json and en.json:
- dashboard.defaultTitle
- All health status and benchmark keys already existed

Build verified successfully. Remaining 31 components to be refactored in subsequent commits.

https://claude.ai/code/session_4f888c33-8937-4db8-8a9d-ddc9ac51a725

frontend/components/ProgressStepper.tsx

@@ -1,6 +1,7 @@
 import React from 'react';
 import { motion } from 'framer-motion';
 import { Check, Package, Upload, BarChart3 } from 'lucide-react';
+import { useTranslation } from 'react-i18next';
 import clsx from 'clsx';
 
 interface Step {
@@ -13,13 +14,15 @@ interface ProgressStepperProps {
   currentStep: number;
 }
 
-const steps: Step[] = [
-  { id: 1, label: 'Seleccionar Tier', icon: Package },
-  { id: 2, label: 'Subir Datos', icon: Upload },
-  { id: 3, label: 'Ver Resultados', icon: BarChart3 },
-];
-
 const ProgressStepper: React.FC<ProgressStepperProps> = ({ currentStep }) => {
+  const { t } = useTranslation();
+
+  const steps: Step[] = [
+    { id: 1, label: t('stepper.selectTier'), icon: Package },
+    { id: 2, label: t('stepper.uploadData'), icon: Upload },
+    { id: 3, label: t('stepper.viewResults'), icon: BarChart3 },
+  ];
+
   return (
     <div className="w-full max-w-3xl mx-auto mb-8">
       <div className="relative flex items-center justify-between">