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_detailed.sh

@@ -0,0 +1,35 @@
+#!/nix/store/ypgcdmzzlgnrmdcsq72c3dxz651jg9zc-bash-5.3p3/bin/bash
+
+# Start the server in the background
+./markdown-editor --data-dir ./test/data --port 8082 --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 -i http://127.0.0.1:8082/api/files)
+echo "$response"
+echo ""
+
+# Extract just the body (skip headers)
+body=$(echo "$response" | awk 'NR>1 && $0 !~ /^[A-Za-z]/ {print; next} /^[A-Za-z]/ {exit}')
+echo "Body: '$body'"
+echo ""
+
+# Test if it's valid JSON
+echo "Checking if response is valid JSON..."
+if echo "$body" | node -e "console.log(JSON.parse(require('fs').readFileSync(0, 'utf8')))" > /dev/null 2>&1; then
+    echo "✓ Response is valid JSON"
+    echo "$body" | node -e "console.log(JSON.parse(require('fs').readFileSync(0, 'utf8')))"
+else
+    echo "✗ Response is NOT valid JSON"
+    echo "Response was: '$body'"
+fi
+
+# Kill the server
+kill $SERVER_PID 2>/dev/null
+wait $SERVER_PID 2>/dev/null
+
+echo "Test completed"