feat(ui): list reordering, quick actions, and resizable modals

- Drag-to-reorder user lists in the sidebar, persisted via a new
  list sort_order column (AddListSortOrder migration, backfilled by
  creation time) and ListRepository.ReorderAsync
- "Open in Explorer" / "Open in Terminal" context-menu actions on lists
- "Clear all completed" button on the Tasks island
- Inline-edit subtask titles (empty text deletes the step) and
  click-to-copy task ID in the Details island
- Make modal and planning windows resizable (BorderOnly decorations
  with min sizes) instead of fixed-size borderless

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -126,8 +126,17 @@
               <Binding Path="IsShowingCompleted"/>
             </MultiBinding>
           </StackPanel.IsVisible>
-          <TextBlock Classes="eyebrow section-label"
-                     Text="{Binding CompletedHeader}" Margin="14,14,14,6"/>
+          <Grid ColumnDefinitions="*,Auto" Margin="14,14,14,6">
+            <TextBlock Grid.Column="0" Classes="eyebrow section-label"
+                       Text="{Binding CompletedHeader}" VerticalAlignment="Center"/>
+            <Button Grid.Column="1" Classes="icon-btn"
+                    Command="{Binding ClearCompletedCommand}"
+                    ToolTip.Tip="Clear all completed"
+                    VerticalAlignment="Center">
+              <PathIcon Data="{StaticResource Icon.Trash}" Width="13" Height="13"
+                        Foreground="{DynamicResource BloodBrush}"/>
+            </Button>
+          </Grid>
           <ItemsControl ItemsSource="{Binding CompletedItems}">
             <ItemsControl.ItemTemplate>
               <DataTemplate DataType="vm:TaskRowViewModel">