Add main entry point for Blurb Photo Book Editor

- Implemented the main function to launch the editor.
- Added error handling for missing PySide6 dependency.
- Included instructions for installing required packages.

requirements.txt

@@ -8,6 +8,9 @@
 
 # --- Direct dependencies ---
 
+# Windows desktop editor UI (official Qt for Python bindings).
+pyside6==6.11.0
+
 # Measure images, read EXIF orientation, do the cover-crop/contain resize.
 pillow==12.3.0
 
@@ -35,4 +38,4 @@ charset-normalizer==3.5.2
 # build tooling
 packaging==26.3
 setuptools==84.0.0
-wheel==0.48.0
+wheel==0.48.0