fix(bulk-delete): snapshot targets and bail on first failure

executeDeleteCompleted and executeDeleteCompletedSubtasks iterated over
the reactive completedTasks/completedSubtasks lists with no error
handling: the array shrinks with every successful delete, skipping
subsequent entries, and a failed delete silently left a half-deleted
state. Snapshot the target list up front and abort as soon as a delete
returns false — matching the subtask-cascade path.

apps/tauri/src/lib/components/TaskDetailView.svelte

@@ -120,7 +120,12 @@
   async function executeDeleteCompletedSubtasks() {
     confirmDeleteCompleted = false;
     showSubtaskMenu = false;
-    for (const s of completedSubtasks) await app.deleteTask(s.id);
+    // Snapshot — completedSubtasks is reactive and shrinks as we delete.
+    // Bail on first failure so we don't silently leave a partial delete.
+    const targets = [...completedSubtasks];
+    for (const s of targets) {
+      if (!(await app.deleteTask(s.id))) return;
+    }
   }
 
   function handleSubtaskMenuClickOutside(e: MouseEvent) {

apps/tauri/src/lib/screens/TasksScreen.svelte

@@ -100,7 +100,12 @@
 
   async function executeDeleteCompleted() {
     confirmDeleteCompleted = false;
-    for (var t of app.completedTasks) await app.deleteTask(t.id);
+    // Snapshot targets first — deletes mutate app.completedTasks reactively.
+    // Bail on first failure so we don't silently leave a partial delete.
+    const targets = [...app.completedTasks];
+    for (const t of targets) {
+      if (!(await app.deleteTask(t.id))) return;
+    }
   }
 
   function promptDeleteList() {