Previously, there was no way to tell who closed (or reopened) a poll —
neither the staff action log nor the poll itself recorded it, as raised
in [meta](https://meta.discourse.org/t/who-closed-the-poll/405495).
This change logs `poll_closed`/`poll_opened` staff actions (auto-close
attributed to the system user) and stores `closed_by`/`closed_at` on the
poll, surfacing "Closed by @user" in the poll UI and making the closer
queryable.
---
Also includes a small fix: the poll buttons dropdown rendered a
`<dropdown.divider />` after every item, leaving a stray trailing border
below the last option; it now renders dividers only between items.
**Screenshot**
<img width="1400" height="1200" alt="image"
src="https://github.com/user-attachments/assets/fd04353a-eac6-448d-904e-47b8e5f8f14f"
/>