Fixes#3067.
<hive-jobq-graph> gains a `cancellable` attribute: any non-terminal
node (Pending/Running/Finishing) gets a small cancel button, and a
click dispatches `hive-jobq-graph-cancel` (`detail: { id }`) rather
than POSTing anything itself -- which endpoint actually cancels a
node is the host's domain concept, same "push data out, host decides"
shape `hive-jobq-graph-update` already uses.
builds.js turns it on for R3BU1LD QU3U3, confirms via themedConfirm,
then POSTs the existing `/api/rebuild-queue/{id}/cancel` endpoint.
No manual refresh needed -- cancelling flips node state, which
already fires rebuild_queue_changed over SSE, and the page's existing
handler for that tick already calls jobqGraphEl.refresh().
Also removed ~130 lines of dead `.rqe-*` CSS in system-sections.css
left over from the bespoke pre-<hive-jobq-graph> queue renderer
(confirmed zero JS references before deleting each rule; kept the
still-used `.rqe-kind`/`.rqe-agent`/`.rqe-source*`).
docs/web-ui/dashboard.md's R3BU1LD QU3U3 section updated to match
current behaviour (cancel button, waits-on text instead of the old
"no per-node actions" note, sibling order no longer implies anything
since deps render as text not a reordered rail).
aa14339be7