fix(tauri): surface errors from toggle_task cascade

When a parent task was toggled, `update_task` failures on child tasks
were silently swallowed with `let _ = ...`, leaving subtasks out of
sync with the parent's status and giving the user no feedback. Map the
error and propagate so the UI can show it and the user can retry.

apps/tauri/src-tauri/src/lib.rs

@@ -495,7 +495,9 @@ fn toggle_task(
                 TaskStatus::Backlog => child.uncomplete(),
                 TaskStatus::Completed => child.complete(),
             }
-            let _ = repo.update_task(lid, child);
+            let child_id = child.id;
+            repo.update_task(lid, child)
+                .map_err(|e| format!("Failed to cascade to subtask {}: {}", child_id, e))?;
         }
     }
     Ok(task)