fix: correct folder path validation error handling in AddLibraryFolder

- Fix scoping issue with err variable in os.Stat check
- Properly check for non-existent vs inaccessible folders
- Use reassignment (=) instead of declaration (:=) since err already declared

internal/handlers/library.go

@@ -175,10 +175,11 @@ func (h *LibraryHandler) AddLibraryFolder(c echo.Context) error {
 	}
 
 	// Validate that folder path exists and is accessible
-	if _, err := os.Stat(req.FolderPath); os.IsNotExist(err) {
-		return c.JSON(http.StatusBadRequest, map[string]string{"error": "folder path does not exist"})
-	}
+	_, err = os.Stat(req.FolderPath)
 	if err != nil {
+		if os.IsNotExist(err) {
+			return c.JSON(http.StatusBadRequest, map[string]string{"error": "folder path does not exist"})
+		}
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "folder is not accessible"})
 	}