fix: resolve JSON.parse error in frontend API calls

- Add error handling for JSON encoding in all backend handlers
- Add CORS support to backend server
- Add proxy configuration for frontend development server
- Improve error handling in frontend API calls
- Build frontend to create static files for backend serving
- Add comprehensive error messages and user feedback

Fixes issue where frontend received malformed JSON responses from backend API.

backend/test/test_api.sh

@@ -0,0 +1,28 @@
+#!/nix/store/ypgcdmzzlgnrmdcsq72c3dxz651jg9zc-bash-5.3p3/bin/bash
+
+# Start the server in the background
+./markdown-editor --data-dir ./test/data --port 8081 --host 127.0.0.1 &
+SERVER_PID=$!
+
+# Give the server time to start
+sleep 2
+
+# Test the /api/files endpoint
+echo "Testing GET /api/files..."
+response=$(curl -s http://127.0.0.1:8081/api/files)
+echo "Response: $response"
+echo ""
+
+# Test if it's valid JSON
+if echo "$response" | python3 -m json.tool > /dev/null 2>&1; then
+    echo "✓ Response is valid JSON"
+else
+    echo "✗ Response is NOT valid JSON"
+    echo "Response was: $response"
+fi
+
+# Kill the server
+kill $SERVER_PID 2>/dev/null
+wait $SERVER_PID 2>/dev/null
+
+echo "Test completed"