feat(web): click Executions' Started time to toggle relative/absolute

Both renderings are pre-rendered server-side into a <button>'s
data-relative/data-absolute attributes; app.js swaps textContent (and the
hover title, to the opposite value) on click, delegated on document.body
so it survives the card's own 5s htmx poll without needing to re-wire
after every swap. Click again to swap back.

docs/WEB-UI.md

@@ -359,6 +359,14 @@ table is wide as a result — 13 columns — and scrolls horizontally inside the
 card's own `.table-wrap` rather than the page, same as the other wide tables
 (Templates, Services).
 
+**Started is click-to-toggle.** It renders relative ("2h ago") by default;
+clicking swaps to the absolute local timestamp, and clicking again swaps
+back. Both strings are pre-rendered server-side into the button's
+`data-relative`/`data-absolute` attributes, so the swap (`app.js`, delegated
+on `document.body`) is instant and needs no request — and keeps working
+after the card's own 5s poll replaces the row, since the listener isn't on
+the button itself.
+
 Runs come from Gitea's Actions API: `/api/v1/admin/actions/runs` (every repo
 on the instance, admin token) with `/api/v1/user/actions/runs` as the
 non-admin fallback. Results are cached for 10s, so the card's 5s refresh costs