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/Models/SettingsDto.cs

@@ -168,6 +168,20 @@ namespace ImageCatalog_2.Models
         [XmlElement("MarchioAggiungi")]
         public bool AddLogo { get; set; }
 
+        // Color-key transparency settings
+        [JsonPropertyName("TransparentColor")]
+        [XmlElement("ColoreTrasparente")]
+        public string TransparentColor { get; set; } = "#FFFFFF";
+
+        [JsonPropertyName("UseTransparentColor")]
+        [XmlElement("UsaColoreTrasparente")]
+        public bool UseTransparentColor { get; set; } = false;
+
+        // Selected image processing library (e.g., "System.Graphics" or "ImageSharp")
+        [JsonPropertyName("ImageLibrary")]
+        [XmlElement("ImageLibrary")]
+        public string ImageLibrary { get; set; } = "System.Graphics";
+
         // Options
         [JsonPropertyName("ForceJpeg")]
         [XmlElement("GeneraleForzaJpg")]