Implement theme management and settings dialog in PLib video library manager. Add theme service for light/dark/system themes, integrate settings dialog for user preferences, and update app configuration to include appearance options. Enhance thumbnail caching with new methods for cache size and clearing. Update README.md to reflect new features and settings.

src/PLib.Desktop/Views/MainWindow.axaml

@@ -145,6 +145,10 @@
             <icons:MaterialIcon Kind="ThemeLightDark" Width="17" Height="17" />
           </Button>
 
+          <Button Command="{Binding OpenSettingsCommand}" ToolTip.Tip="Настройки">
+            <icons:MaterialIcon Kind="CogOutline" Width="17" Height="17" />
+          </Button>
+
           <Button Classes="Primary" Command="{Binding AddFolderCommand}">
             <StackPanel Orientation="Horizontal" Spacing="7">
               <icons:MaterialIcon Kind="FolderPlusOutline" Width="17" Height="17" />