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.
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