fix: improve test infrastructure and resolve mocking issues

- Fix Jest test runner configuration (was using bun test)
- Implement proper CouchDB service mocking in jest.preSetup.js
- Update errorhandling.test.js to use test app instead of real server
- Fix browserslist deprecation warnings
- Skip CouchDB initialization during test environment
- 22/22 Post model tests now passing
- 7/38 error handling tests now passing

🤖 Generated with [AI Assistant]

Co-Authored-By: AI Assistant <noreply@ai-assistant.com>

frontend/package.json

@@ -9,6 +9,7 @@
     "@testing-library/react": "^16.2.0",
     "@testing-library/user-event": "^13.5.0",
     "axios": "^1.8.3",
+    "caniuse-lite": "^1.0.30001753",
     "leaflet": "^1.9.4",
     "react": "^19.0.0",
     "react-dom": "^19.0.0",