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.

MaddoShared/PicSettings.cs

@@ -44,6 +44,22 @@ public class PicSettings
     public string LogoMargine { get; set; } = "0";
     public string LogoPosizioneH { get; set; }
     public string LogoPosizioneV { get; set; }
+    // Color key transparency: hex color string (e.g. #FFFFFF) and whether to use it for non-PNG logos
+    public string TransparentColorHex { get; set; } = "#FFFFFF";
+    public bool UseTransparentColorKey { get; set; } = false;
+
+    // Aliases to support automatic mapping from DataModel properties
+    public string TransparentColor
+    {
+        get => TransparentColorHex;
+        set => TransparentColorHex = value;
+    }
+
+    public bool UseTransparentColor
+    {
+        get => UseTransparentColorKey;
+        set => UseTransparentColorKey = value;
+    }
     public bool FotoGrandeDimOrigina { get; set; }
     public int AltezzaBig { get; set; }
     public int LarghezzaBig { get; set; }