Replace stub QMessageBox in Add Silo and Edit Origin buttons with
actual calls to the Silo_Settings command, so users can configure
their Silo connection from the origin manager dialog.
- Create OriginManagerDialog with list of configured origins
- Show connection status for remote origins
- Allow setting default origin
- Add/Edit/Remove buttons (Add/Edit show placeholder for now)
- Wire up 'Manage Origins...' button in OriginSelectorWidget
- Prevent removal of built-in local origin
Part of Issue #15: Multi-instance Silo configuration UI
