fix(ui): avoid int CommandParameter cast crash in usage monitor presets

RelayCommand<int> casts its object parameter directly rather than
converting it, so a string CommandParameter ("7"/"30") would throw
InvalidCastException at runtime. Split into two parameterless commands.

tests/ClaudeDo.Ui.Tests/ViewModels/UsageMonitorModalViewModelTests.cs

@@ -258,7 +258,7 @@ public class UsageMonitorModalViewModelTests
         await vm.LoadAsync();
         var callsAfterInitialLoad = worker.ModelUsageCalls;
 
-        await vm.SetPresetCommand.ExecuteAsync(30);
+        await vm.SetPreset30DaysCommand.ExecuteAsync(null);
 
         Assert.Equal(callsAfterInitialLoad + 1, worker.ModelUsageCalls);
         Assert.Equal(callsAfterInitialLoad + 1, worker.TaskUsageCalls);