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_api.sh

@@ -0,0 +1,82 @@
+#!/nix/store/ypgcdmzzlgnrmdcsq72c3dxz651jg9zc-bash-5.3p3/bin/bash
+
+# Create test data directory
+mkdir -p test/data
+
+# Start the server in the background
+./markdown-editor --data-dir test/data --port 8080 --host 127.0.0.1 > /tmp/server.log 2>&1 &
+SERVER_PID=$!
+
+# Wait for server to start
+echo "Waiting for server to start..."
+sleep 3
+
+# Check if server is running
+if ! ps -p $SERVER_PID > /dev/null; then
+    echo "Server failed to start"
+    cat /tmp/server.log
+    exit 1
+fi
+
+echo "Server is running (PID: $SERVER_PID)"
+echo "Testing API endpoints..."
+echo ""
+
+# Test 1: GET /api/files (empty)
+echo "Test 1: GET /api/files (no files)"
+response=$(curl -s http://127.0.0.1:8080/api/files)
+echo "Response: $response"
+if echo "$response" | grep -q '"files"'; then
+    echo "✓ Response contains 'files' key"
+else
+    echo "✗ Response does not contain 'files' key"
+fi
+echo ""
+
+# Test 2: Create a file
+echo "Test 2: POST /api/files (create test.md)"
+response=$(curl -s -X POST http://127.0.0.1:8080/api/files \
+    -H "Content-Type: application/json" \
+    -d '{"filename":"test.md","content":"# Test Content"}')
+echo "Response: $response"
+if echo "$response" | grep -q '"message"'; then
+    echo "✓ Response contains 'message' key"
+else
+    echo "✗ Response does not contain 'message' key"
+fi
+echo ""
+
+# Test 3: GET /api/files (with file)
+echo "Test 3: GET /api/files (with file)"
+response=$(curl -s http://127.0.0.1:8080/api/files)
+echo "Response: $response"
+if echo "$response" | grep -q '"files"'; then
+    echo "✓ Response contains 'files' key"
+    if echo "$response" | grep -q '"test.md"'; then
+        echo "✓ Response contains 'test.md'"
+    else
+        echo "✗ Response does not contain 'test.md'"
+    fi
+else
+    echo "✗ Response does not contain 'files' key"
+fi
+echo ""
+
+# Test 4: GET /api/files/test.md
+echo "Test 4: GET /api/files/test.md"
+response=$(curl -s http://127.0.0.1:8080/api/files/test.md)
+echo "Response: $response"
+if echo "$response" | grep -q '"content"'; then
+    echo "✓ Response contains 'content' key"
+else
+    echo "✗ Response does not contain 'content' key"
+fi
+echo ""
+
+# Kill the server
+echo "Stopping server..."
+kill $SERVER_PID
+wait $SERVER_PID 2>/dev/null
+
+echo ""
+echo "All tests completed!"