refactor(app): split MainWindowViewModel into partial files by area
1300-line monolith → 7 focused files (max 359 lines each):
MainWindowViewModel.cs core fields, constructor, lifecycle
MainWindowViewModel.Wizard.cs setup wizard and wallet open/close flows
MainWindowViewModel.Settings.cs language, unit, overlay flags
MainWindowViewModel.Sync.cs server config, connection, sync loop
MainWindowViewModel.Send.cs send panel
MainWindowViewModel.Contacts.cs contact management
MainWindowViewModel.Receive.cs balance display, QR, addresses, tx details
No behaviour change, no XAML change, 127/127 tests pass.
davide
2ddc0f920c