feat(tasks): PR 1.5 — return refreshed subtask + parent from completion

Change complete_subtask_and_check_parent to return
(TaskRow, Option<TaskRow>): the refreshed subtask, plus the parent when
the cascade auto-completed it. Both rows are read inside the same
transaction so the frontend never observes partial state.

Wrap the cmd in a CompleteSubtaskResult { updatedSubtask,
autoCompletedParent } DTO. MicroSteps now applies the refreshed subtask
locally and, when present, calls a new replaceTaskFromDto helper on the
Tasks store so the parent's done/doneAt/badge counts update without a
follow-up list_tasks_cmd round-trip.

Test: complete_subtask_returns_updated_subtask_and_optional_parent
covers both paths (sibling-pending → None, last-sibling → Some(parent)).
All 61 kon-storage tests pass; svelte-check clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/stores/page.svelte.ts

@@ -432,6 +432,21 @@ function applyLocalTaskUpdate(id: string, updates: Partial<TaskEntry>) {
   }
 }
 
+/**
+ * PR 1.5: replace an in-store task with a fresh DTO from the backend
+ * (e.g. the auto-completed parent returned by `complete_subtask_cmd`).
+ * No-op if the task is not in the store — that's expected when a parent
+ * has already been pruned client-side. Broadcasts on success so other
+ * windows pick up the new state.
+ */
+export function replaceTaskFromDto(row: TaskDto) {
+  const idx = tasks.findIndex((task) => task.id === row.id);
+  if (idx >= 0) {
+    tasks[idx] = mapTaskRow(row);
+    broadcastTasks();
+  }
+}
+
 export async function updateTask(id: string, updates: TaskUpdate) {
   if (!hasTauriRuntime()) {
     applyLocalTaskUpdate(id, updates as Partial<TaskEntry>);