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/locales/en.json

@@ -131,6 +131,7 @@
   },
   "dashboard": {
     "title": "Diagnostic Dashboard",
+    "defaultTitle": "DEMO CLIENT - Beyond CX Analytics",
     "viewDashboard": "View Diagnostic Dashboard",
     "generateAnalysis": "Generate Analysis",
     "analyzing": "Analyzing...",