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.

main.py

@@ -0,0 +1,20 @@
+"""Launch the Blurb Photo Book Editor."""
+
+from __future__ import annotations
+
+
+def main() -> int:
+    try:
+        from blurb_editor import run
+    except ImportError as exc:
+        if exc.name and exc.name.startswith("PySide6"):
+            raise SystemExit(
+                "PySide6 is not installed in this Python environment. "
+                "Run .\\.venv\\Scripts\\python.exe -m pip install -r requirements.txt first."
+            ) from exc
+        raise
+    return run()
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())