Restrict file dialog to .mflow files only and return to login on load error

- Remove "All files (*)" option from file dialog, only allow .mflow files
- Change error handling to return to login page instead of opening empty pipeline
- Update error message to be more specific about file format requirements
- Properly clean up dashboard window when file load fails

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

ui/windows/login.py

@@ -380,7 +380,7 @@ class DashboardLogin(QWidget):
             self,
             "Open Pipeline File",
             self.settings.get_default_project_location(),
-            "Pipeline files (*.mflow);;All files (*)"
+            "Pipeline files (*.mflow)"
         )
         
         if file_path:
@@ -436,10 +436,15 @@ class DashboardLogin(QWidget):
                 except Exception as e:
                     QMessageBox.warning(
                         self,
-                        "File Load Warning",
+                        "File Load Error",
                         f"Could not load pipeline file: {e}\n\n"
-                        "Opening with empty pipeline instead."
+                        "Please select a valid .mflow file."
                     )
+                    # Close the dashboard window and return to login page
+                    self.dashboard_window.close()
+                    self.dashboard_window = None
+                    self.show()  # Show the login window again
+                    return
             
             self.dashboard_window.show()
             self.hide()  # Hide the login window