Replaced multiple mutually-exclusive boolean properties for thumbnail text options with a single enum (`ThumbnailOption`) in the data model. Updated WinForms UI to use a ComboBox instead of radio buttons for selecting thumbnail mode. Adjusted designer, mapping profile, settings DTO, and settings service for enum support and backward compatibility. Simplified thumbnail generation logic and improved maintainability by ensuring only one mode can be selected at a time.
- 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.
Dialogs now remember last-used folders for source, destination, logo, and settings files by storing these paths in a user preferences file under LocalApplicationData. Preferences are saved on form close, reducing unnecessary writes. SettingsService now uses a temporary ParametriSetup for settings files to avoid polluting user preferences. Error handling ensures preference save failures do not disrupt the user. This separation improves user experience and keeps user preferences distinct from project settings.
