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-tauri/src/commands/tasks.rs

@@ -378,14 +378,29 @@ pub async fn list_subtasks_cmd(
         .map_err(|e| e.to_string())
 }
 
+/// Return shape for `complete_subtask_cmd`. The frontend uses
+/// `updated_subtask` to refresh the row in MicroSteps and, when present,
+/// uses `auto_completed_parent` to swap the parent in the Tasks store
+/// without a round-trip refetch (PR 1.5).
+#[derive(Debug, Clone, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct CompleteSubtaskResult {
+    pub updated_subtask: TaskDto,
+    pub auto_completed_parent: Option<TaskDto>,
+}
+
 #[tauri::command]
 pub async fn complete_subtask_cmd(
     state: tauri::State<'_, AppState>,
     subtask_id: String,
-) -> Result<(), String> {
-    db_complete_subtask(&state.db, &subtask_id)
+) -> Result<CompleteSubtaskResult, String> {
+    let (subtask_row, parent_row) = db_complete_subtask(&state.db, &subtask_id)
         .await
-        .map_err(|e| e.to_string())
+        .map_err(|e| e.to_string())?;
+    Ok(CompleteSubtaskResult {
+        updated_subtask: TaskDto::from(subtask_row),
+        auto_completed_parent: parent_row.map(TaskDto::from),
+    })
 }
 
 /// Phase 8: daily completion counts for the Tasks-page badge and the