Fix crash when switching converter categories

[veler]

Fix crash when switching converter categories

Navigating between Converter categories (e.g. Currency -> Temperature, Temperature -> Weight and mass) intermittently crashed the app.

Root cause: on a category change UnitConverterViewModel rebuilds the bound Units collection in place and then assigns Unit1/Unit2, which x:Bind pushes to each ComboBox's SelectedItem synchronously inside the NavigationView selection handler. WinUI's ComboBox validates SelectedItem against its own item collection, which it refreshes from the ItemsSource change on a later, deferred pass. When the selection is applied before that refresh completes, Selector.put_SelectedItem throws ArgumentException (0x80070057) which, being unhandled in the XAML delegate, fail-fasts the process. It's timing-dependent, so it reproduces intermittently and vanishes under a debugger.

The C++/CX version never hit this because classic {Binding} routed selection through the AlwaysSelectedCollectionView (ICollectionView) currency, which defers gracefully. That currency integration does not reproduce in the C# projection: reverting to {Binding} makes the ComboBox write its default (index 0) selection back through the TwoWay binding, corrupting the engine's from/to units instead. SelectedIndex binding, a single Reset notification, an atomic collection swap, and a deferred dispatcher push were all verified on-device not to close the race, because the lag lives inside the control.

Contain the specific transient at the assignment site (AssignSelectedUnit): the Unit1/Unit2 backing fields are set before the throw and the ComboBox reconciles its selection on the next layout pass, so the displayed selection is correct. Verified: 0 crashes across 110+ category navigations; conversions (e.g. 100 C = 212 F) and dropdown selection continue to work.