fix: Resolve i18n dependency issues for different environments

- Update i18n package versions to stable, compatible versions
- Add install-i18n.sh script for easy setup in any environment
- Create comprehensive I18N_SETUP.md troubleshooting guide
- Use legacy-peer-deps flag to avoid npm conflicts
- Provide clear instructions for Docker and local environments

This ensures the German translation feature works correctly
across different development and deployment environments.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/package.json

@@ -16,13 +16,13 @@
     "axios": "^1.6.5",
     "chart.js": "^4.4.1",
     "date-fns": "^3.2.0",
-    "i18next": "^25.3.2",
-    "i18next-browser-languagedetector": "^8.2.0",
+    "i18next": "^23.7.6",
+    "i18next-browser-languagedetector": "^7.2.0",
     "react": "^18.2.0",
     "react-chartjs-2": "^5.2.0",
     "react-dom": "^18.2.0",
     "react-hook-form": "^7.48.2",
-    "react-i18next": "^15.6.1",
+    "react-i18next": "^13.5.0",
     "react-router-dom": "^6.21.1",
     "react-scripts": "5.0.1",
     "typescript": "^5.3.3",