fix: replace obsolete EnableHotReload with UseStudio in TodoApp.Uno

[adrianhall]

Summary

App.xaml.cs calls the now-obsolete WindowExtensions.EnableHotReload(Window) API, producing a UNO0008 warning on every todoapp-uno / * CI job build.

Fix

Per UNO0008, Uno Platform 5.5+ deprecated Window.EnableHotReload() for Uno.Sdk-style projects (which TodoApp.Uno.csproj already is) in favor of Window.UseStudio() — a drop-in, same-signature replacement. Uno's own solution templates use exactly this pattern (MainWindow.UseStudio(); inside #if DEBUG).

 #if DEBUG
-        MainWindow.EnableHotReload();
+        MainWindow.UseStudio();
 #endif

Low risk, purely a hot-reload developer-experience API swap with no runtime behavior change for a normal (non-hot-reload) build/run.

Testing

• dotnet restore samples/todoapp/TodoApp.Uno/TodoApp.Uno/TodoApp.Uno.csproj -p:TargetFramework=net10.0-desktop
• dotnet build samples/todoapp/TodoApp.Uno/TodoApp.Uno/TodoApp.Uno.csproj -f net10.0-desktop --configuration Debug --no-restore
  • Build succeeded, 0 errors, and the UNO0008 warning no longer appears.
  • Remaining warnings (NU1510 x3, CS8618 x2, CS8622, Uno0001) are pre-existing and unrelated to this change.
• Other heads (android/ios/maccatalyst/windows/browserwasm) not built locally (missing workloads/Xcode/Windows runner here); same code path executes on every head, so CI's build-samples-todoapp-uno.yml matrix will validate them on this PR.

Fixes #536