hive-agent: web UI "mark done" reports a failed todo store instead of success

post_mark_todos_done dropped every dial failure and Response::Err and
answered 200 {"acked":0}, so the todos panel showed "marked done" and
cleared its checkboxes while the todos stayed. Any failing id now fails
the request with a 500 naming how many ids failed; the panel already
shows non-OK bodies as "failed: ...".

docs/web-ui/agent.md

@@ -160,6 +160,9 @@ select-all / select-none / `✓ mark done` bulk row above the list POSTs
 the checked ids to `POST /api/todos/mark-done`, which dismisses them
 from the harness-local store (same effect as `cancel_loose_end(kind:
 "todo")`, just from the web UI instead of the agent's own tool calls).
+If the todo store errors or doesn't answer for any id, the endpoint returns
+500 naming how many ids failed, and the panel shows `failed: …` instead of
+`✓ marked done`.
 
 ## Live view