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/handlers/handlers.go

@@ -31,9 +31,13 @@ func (h *Handlers) ListFiles() http.HandlerFunc {
 		}
 
 		w.Header().Set("Content-Type", "application/json")
-		json.NewEncoder(w).Encode(map[string]interface{}{
+		if err := json.NewEncoder(w).Encode(map[string]interface{}{
 			"files": files,
-		})
+		}); err != nil {
+			h.logger.Errorf("Failed to encode response: %v", err)
+			h.writeError(w, http.StatusInternalServerError, "failed to encode response")
+			return
+		}
 	}
 }
 
@@ -62,9 +66,13 @@ func (h *Handlers) CreateFile() http.HandlerFunc {
 		}
 
 		w.Header().Set("Content-Type", "application/json")
-		json.NewEncoder(w).Encode(map[string]string{
+		if err := json.NewEncoder(w).Encode(map[string]string{
 			"message": "file created successfully",
-		})
+		}); err != nil {
+			h.logger.Errorf("Failed to encode response: %v", err)
+			h.writeError(w, http.StatusInternalServerError, "failed to encode response")
+			return
+		}
 	}
 }
 
@@ -86,10 +94,14 @@ func (h *Handlers) GetFile() http.HandlerFunc {
 		}
 
 		w.Header().Set("Content-Type", "application/json")
-		json.NewEncoder(w).Encode(map[string]interface{}{
+		if err := json.NewEncoder(w).Encode(map[string]interface{}{
 			"filename": filename,
 			"content":  string(content),
-		})
+		}); err != nil {
+			h.logger.Errorf("Failed to encode response: %v", err)
+			h.writeError(w, http.StatusInternalServerError, "failed to encode response")
+			return
+		}
 	}
 }
 
@@ -120,9 +132,13 @@ func (h *Handlers) UpdateFile() http.HandlerFunc {
 		}
 
 		w.Header().Set("Content-Type", "application/json")
-		json.NewEncoder(w).Encode(map[string]string{
+		if err := json.NewEncoder(w).Encode(map[string]string{
 			"message": "file updated successfully",
-		})
+		}); err != nil {
+			h.logger.Errorf("Failed to encode response: %v", err)
+			h.writeError(w, http.StatusInternalServerError, "failed to encode response")
+			return
+		}
 	}
 }
 
@@ -143,16 +159,23 @@ func (h *Handlers) DeleteFile() http.HandlerFunc {
 		}
 
 		w.Header().Set("Content-Type", "application/json")
-		json.NewEncoder(w).Encode(map[string]string{
+		if err := json.NewEncoder(w).Encode(map[string]string{
 			"message": "file deleted successfully",
-		})
+		}); err != nil {
+			h.logger.Errorf("Failed to encode response: %v", err)
+			h.writeError(w, http.StatusInternalServerError, "failed to encode response")
+			return
+		}
 	}
 }
 
 func (h *Handlers) writeError(w http.ResponseWriter, status int, message string) {
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(status)
-	json.NewEncoder(w).Encode(map[string]string{
+	if err := json.NewEncoder(w).Encode(map[string]string{
 		"error": message,
-	})
+	}); err != nil {
+		h.logger.Errorf("Failed to encode error response: %v", err)
+		w.Write([]byte(`{"error": "failed to encode error response"}`))
+	}
 }

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 {