Add welcome wizard, desktop entry, and Nautilus extension

First-run welcome dialog with skill level and output location setup.
Desktop entry file for GNOME app launcher integration.
Nautilus Python extension with dynamic 'Process with Pixstrip' submenu
that reads both built-in and user presets.

data/live.lashman.Pixstrip.desktop

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Name=Pixstrip
+Comment=Batch image processor - resize, convert, compress, and more
+Exec=pixstrip-gtk %F
+Icon=live.lashman.Pixstrip
+Terminal=false
+Type=Application
+Categories=Graphics;ImageProcessing;
+MimeType=image/jpeg;image/png;image/webp;image/avif;image/gif;image/tiff;image/bmp;
+Keywords=image;photo;resize;convert;compress;batch;metadata;strip;watermark;rename;
+StartupNotify=true