Replace magic strings with enums in sorting logic

Create new SortingMethod enum alongside deprecated FolderSortingMethod for
backward compatibility. Replace all magic string direction values ('asc'/'desc')
with SortingOrder enum constants. Add validateDirection() method to gracefully
handle invalid direction values by defaulting to descending order. Include
comprehensive test coverage for invalid sorting inputs.

Author: arukompas

config/log-viewer.php

@@ -1,6 +1,6 @@
 <?php
 
-use Opcodes\LogViewer\Enums\FolderSortingMethod;
+use Opcodes\LogViewer\Enums\SortingMethod;
 use Opcodes\LogViewer\Enums\SortingOrder;
 use Opcodes\LogViewer\Enums\Theme;
 
@@ -283,13 +283,13 @@
         'use_local_storage' => true,
 
         // Method to sort the folders. Other options: `Alphabetical`, `ModifiedTime`
-        'folder_sorting_method' => FolderSortingMethod::ModifiedTime,
+        'folder_sorting_method' => SortingMethod::ModifiedTime,
 
         // Order to sort the folders. Other options: `Ascending`, `Descending`
         'folder_sorting_order' => SortingOrder::Descending,
 
         // Method for sorting log-files into directories. Other options: `Alphabetical`, `ModifiedTime`
-        'file_sorting_method' => FolderSortingMethod::ModifiedTime,
+        'file_sorting_method' => SortingMethod::ModifiedTime,
 
         // Order to sort the logs. Other options: `Ascending`, `Descending`
         'log_sorting_order' => SortingOrder::Descending,