refactor(ui): merge the one-expression converters into SimpleConverters.cs

Six files of three lines each. NotNullToBool is gone entirely — Avalonia
ships ObjectConverters.IsNotNull, so TaskRowView binds that directly.

src/ClaudeDo.Ui/Views/Islands/TaskRowView.axaml

@@ -2,6 +2,7 @@
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              xmlns:vm="using:ClaudeDo.Ui.ViewModels.Islands"
              xmlns:ctl="using:ClaudeDo.Ui.Views.Controls"
+             xmlns:conv="using:Avalonia.Data.Converters"
              xmlns:loc="using:ClaudeDo.Ui.Localization"
              x:Class="ClaudeDo.Ui.Views.Islands.TaskRowView"
              x:DataType="vm:TaskRowViewModel">
@@ -187,7 +188,7 @@
             <!-- Chain-after chip: the head-relative fallback when this row isn't rendered as a
                  rail member (planning child, or the head is filtered out of this view). -->
             <Border Classes="chip chip-tag"
-                    IsVisible="{Binding ChainAfterLabel, Converter={StaticResource NotNullToBool}}">
+                    IsVisible="{Binding ChainAfterLabel, Converter={x:Static conv:ObjectConverters.IsNotNull}}">
               <TextBlock>
                 <Run Text="{loc:Tr tasks.chainAfterPrefix}"/>
                 <Run Text=" "/>