style(ui): add strikethrough and dimming for completed tasks

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/TaskListView.axaml

@@ -33,6 +33,7 @@
                 <DataTemplate x:DataType="vm:TaskItemViewModel">
                     <Grid ColumnDefinitions="Auto,*" Margin="4,4"
                           Background="Transparent"
+                          Opacity="{Binding RowOpacity}"
                           DoubleTapped="OnTaskItemDoubleTapped"
                           PointerPressed="OnTaskItemPointerPressed">
                         <Grid.ContextFlyout>
@@ -75,7 +76,8 @@
                         <!-- Task content -->
                         <StackPanel Grid.Column="1" VerticalAlignment="Center">
                             <TextBlock Text="{Binding Title}" FontWeight="Medium"
-                                       Foreground="{StaticResource TextPrimaryBrush}"
+                                       Foreground="{Binding TitleForeground}"
+                                       TextDecorations="{Binding TitleDecorations}"
                                        TextTrimming="CharacterEllipsis"/>
                             <TextBlock FontSize="11"
                                        Foreground="{StaticResource TextDimBrush}"