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/internal/server/server.go

@@ -10,6 +10,7 @@ import (
 	"markdown-editor/internal/logger"
 	"markdown-editor/internal/storage"
 	"github.com/gorilla/mux"
+	"github.com/rs/cors"
 	"github.com/sirupsen/logrus"
 )
 
@@ -73,6 +74,15 @@ func (s *Server) setupRoutes() {
 		frontendDir = frontendDirEnv
 	}
 	s.router.PathPrefix("/").Handler(http.FileServer(http.Dir(frontendDir)))
+
+	// Enable CORS
+	cors := cors.New(cors.Options{
+		AllowedOrigins:   []string{"http://localhost:3000", "http://localhost:8080"},
+		AllowedMethods:   []string{"GET", "POST", "PUT", "DELETE", "OPTIONS"},
+		AllowedHeaders:   []string{"Content-Type", "Authorization"},
+		AllowCredentials: true,
+	})
+	s.router.Use(cors.Handler)
 }
 
 func (s *Server) serve() error {