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/ViewModels/Islands/TasksIslandViewModel.cs

@@ -487,6 +487,38 @@ public sealed partial class TasksIslandViewModel : ViewModelBase
         TasksChanged?.Invoke(this, EventArgs.Empty);
     }
 
+    [RelayCommand]
+    private async Task ClearCompletedAsync()
+    {
+        if (CompletedItems.Count == 0) return;
+
+        // Delete children before parents so the parent-child FK (Restrict) doesn't
+        // block removing a completed planning parent together with its done children.
+        var toDelete = CompletedItems.OrderByDescending(r => r.IsChild).ToList();
+
+        if (ConfirmAsync is not null)
+        {
+            var ok = await ConfirmAsync($"Clear {toDelete.Count} completed task(s)? This cannot be undone.");
+            if (!ok) return;
+        }
+
+        await using var db = await _dbFactory.CreateDbContextAsync();
+        var repo = new TaskRepository(db);
+        foreach (var row in toDelete)
+        {
+            try
+            {
+                await repo.DeleteAsync(row.Id);
+                Items.Remove(row);
+            }
+            catch { /* still referenced by open child tasks; leave it visible */ }
+        }
+
+        Regroup();
+        UpdateSubtitle();
+        TasksChanged?.Invoke(this, EventArgs.Empty);
+    }
+
     [RelayCommand]
     private async Task ToggleStarAsync(TaskRowViewModel row)
     {