Implement offline merge and review core

src/dataloom/app.py

@@ -1,6 +1,7 @@
-"""Application entry point placeholder for the DataLoom desktop UI."""
+"""Application entry point for the DataLoom desktop UI."""
 
 
 def main() -> int:
-    """Start the desktop application once the UI is implemented."""
-    raise NotImplementedError("DataLoom UI is not implemented yet")
+    """Start the desktop application."""
+    from dataloom.ui.main_window import run
+    return run()