fix(ui): replace deprecated Watermark with PlaceholderText

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/TaskDetailView.axaml

@@ -69,7 +69,7 @@
                         </ItemsControl.ItemTemplate>
                     </ItemsControl>
                     <TextBox Text="{Binding NewTagInput}"
-                             Watermark="Add tag..."
+                             PlaceholderText="Add tag..."
                              Width="100" FontSize="12"
                              BorderThickness="0" Background="Transparent"
                              Padding="4,3"
@@ -83,7 +83,7 @@
             <TextBox Text="{Binding Description}"
                      AcceptsReturn="True" TextWrapping="Wrap" MinHeight="60"
                      Foreground="{StaticResource TextPrimaryBrush}"
-                     Watermark="Add a description..."
+                     PlaceholderText="Add a description..."
                      LostFocus="OnFieldLostFocus"/>
 
             <!-- === READ-ONLY ZONE === -->

src/ClaudeDo.Ui/Views/TaskListView.axaml

@@ -11,7 +11,7 @@
                 BorderThickness="0,1,0,0" BorderBrush="{StaticResource BorderSubtleBrush}">
             <TextBox x:Name="InlineAddBox"
                      Text="{Binding InlineAddTitle, Mode=TwoWay}"
-                     Watermark="+ Add a task..."
+                     PlaceholderText="+ Add a task..."
                      BorderThickness="1"
                      BorderBrush="{StaticResource BorderSubtleBrush}"
                      CornerRadius="8"