fix: validate library folder paths before saving

- Add os package import for file system checks
- Validate that folder paths exist before adding to library
- Check folder accessibility to prevent invalid paths
- Return clear error messages for invalid folders

Improves user experience by catching path errors early

internal/handlers/library.go

@@ -4,6 +4,7 @@ import (
 	"bookmann/internal/database"
 	"bookmann/internal/services"
 	"net/http"
+	"os"
 
 	"github.com/google/uuid"
 	"github.com/jackc/pgx/v5/pgtype"
@@ -173,6 +174,14 @@ func (h *LibraryHandler) AddLibraryFolder(c echo.Context) error {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": err.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"})
+	}
+	if err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "folder is not accessible"})
+	}
+
 	folder, err := h.libraryService.AddLibraryFolder(
 		c.Request().Context(),
 		libraryID,