Add color-key transparency support for logo overlays

- Allow users to select a transparent color for logo images (watermarks) via UI (checkbox, color picker, preview).
- Apply color-key transparency in both System.Drawing and ImageSharp backends: specified color in logo is made fully transparent.
- Persist transparency settings in PicSettings and SettingsDto; bind to DataModel and UI controls.
- Update logo preview to reflect transparency in real time.
- Add option to select image processing library (System.Drawing or ImageSharp) in UI and settings.
- Fix bug in SettingsService parameter loading for int/double/DateTime.
- Fully integrate color-key transparency into image processing and settings serialization.

imagecatalog/DataModel.cs

@@ -30,6 +30,7 @@ namespace ImageCatalog_2
         public ICommand SaveSettingsCommand { get; }
         public ICommand LoadSettingsCommand { get; }
         public ICommand SelectColorCommand { get; }
+        public ICommand SelectTransparentColorCommand { get; }
 
         private readonly ITestService _service;
         private readonly ILogger<DataModel> _logger;
@@ -67,6 +68,7 @@ namespace ImageCatalog_2
             SaveSettingsCommand = new RelayCommand(SaveSettings);
             LoadSettingsCommand = new RelayCommand(LoadSettings);
             SelectColorCommand = new RelayCommand(SelectColor);
+            SelectTransparentColorCommand = new RelayCommand(SelectTransparentColor);
 
             // Load available fonts
             AvailableFonts = LoadAvailableFonts();
@@ -341,6 +343,28 @@ namespace ImageCatalog_2
             }
         }
 
+        private string _transparentColor = "#FFFFFF";
+        public string TransparentColor
+        {
+            get => _transparentColor;
+            set
+            {
+                _transparentColor = value;
+                NotifyPropertyChanged();
+            }
+        }
+
+        private bool _useTransparentColor;
+        public bool UseTransparentColor
+        {
+            get => _useTransparentColor;
+            set
+            {
+                _useTransparentColor = value;
+                NotifyPropertyChanged();
+            }
+        }
+
         // Logo/Watermark settings
         private string _logoFile = "";
         public string LogoFile
@@ -1129,6 +1153,7 @@ namespace ImageCatalog_2
         public event EventHandler SelectColorRequested;
         // Request that the View shows a message to the user (message, caption, icon)
         public event EventHandler<Tuple<string, string, MessageBoxIcon>> ShowMessageRequested;
+        public event EventHandler SelectTransparentColorRequested;
 
         private void SelectSourceFolder(object parameter)
         {
@@ -1160,6 +1185,11 @@ namespace ImageCatalog_2
             SelectColorRequested?.Invoke(this, EventArgs.Empty);
         }
 
+        private void SelectTransparentColor(object parameter)
+        {
+            SelectTransparentColorRequested?.Invoke(this, EventArgs.Empty);
+        }
+
         public async Task SaveSettingsToFileAsync(string filePath)
         {
             await _settingsService.SaveSettingsAsync(filePath, this);