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/Design/IslandStyles.axaml

@@ -826,6 +826,15 @@
     <Setter Property="Opacity"         Value="0.5" />
     <Setter Property="TextDecorations" Value="Strikethrough" />
   </Style>
+  <Style Selector="TextBox.subtask-edit">
+    <Setter Property="Padding"         Value="4,2" />
+    <Setter Property="MinHeight"       Value="0" />
+    <Setter Property="Background"      Value="{StaticResource Surface2Brush}" />
+    <Setter Property="BorderBrush"     Value="{DynamicResource LineBrush}" />
+    <Setter Property="BorderThickness" Value="1" />
+    <Setter Property="CornerRadius"    Value="6" />
+    <Setter Property="Foreground"      Value="{DynamicResource TextBrush}" />
+  </Style>
 
   <!-- ============================================================ -->
   <!-- SECTION LABELS (OVERDUE / TASKS / COMPLETED)                 -->