fix(frontend): resolve file loading, display, and cursor issues

- Fix API to return JSON response for file content instead of plain text
- Fix file display showing [object Object] by properly extracting content field
- Fix infinite save loop by tracking last saved content
- Remove auto-save that was causing cursor jumping on every keystroke
- Add manual save button with disabled state when content unchanged
- Add validation in FileList to prevent undefined filenames
- Improve error handling for file operations

backend/internal/api/server.go

@@ -95,8 +95,13 @@ func (s *Server) handleGetFiles(w http.ResponseWriter, r *http.Request) {
 			s.sendError(w, http.StatusNotFound, err.Error())
 			return
 		}
-		w.Header().Set("Content-Type", "text/markdown")
-		w.Write([]byte(content))
+		// Return file content as JSON
+		response := map[string]string{
+			"name":  filename,
+			"content": content,
+		}
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(response)
 		return
 	}