dashboard: stream the running rebuild's build log inline in the queue

The C0R3 rebuild queue only linked out to the logs page (logs →). Add an
inline live-log panel under the queue that streams the currently-running
rebuild's build output, so the operator watches progress without leaving the
page.

One panel keyed to the running entry's build_log_id (the queue runs one build
at a time), reusing the build-log SSE the logs page already uses
(GET /api/build-logs/id/{id}/stream; frames stdout_append/stderr_append/done).
It lives in its own container (#rebuild-live-log) outside rebuild-queue-section
so the queue's per-row re-render — rows rebuild as the build step advances —
never tears down the open stream; it reconnects only when the running
build_log_id changes and won't reopen a stream that already sent done. Sticky-
bottom scroll, collapsible, live/ok/fail badge, raw download. Hidden when
nothing is building; each row keeps its logs → link for full history.

Frontend-only — no backend change (endpoint + build_log_id already existed).
Closes #1860.

frontend/packages/dashboard/src/core.js

@@ -158,9 +158,10 @@ function rebuildQueueEntryFingerprint(entry, isChild) {
 }
 
 function renderRebuildQueue(s) {
+  const queue = s.rebuild_queue || [];
+  renderRebuildLiveLog(queue);
   const root = $('rebuild-queue-section');
   if (!root) return;
-  const queue = s.rebuild_queue || [];
 
   if (!queue.length) {
     rebuildQueueRowCache.clear();
@@ -294,6 +295,121 @@ function renderQueueEntry(entry, _byId, isChild) {
   return li;
 }
 
+// ─── running-rebuild live log ─────────────────────────────────────────────
+// One persistent live-log panel for the currently-running rebuild (the queue
+// runs one build at a time). Keyed to the running entry's build_log_id and
+// kept in its own container (#rebuild-live-log) so the queue's row re-render
+// — which rebuilds rows as the build step advances — never tears down the open
+// SSE stream. Reuses the build-log stream the logs page BUILD tab uses
+// (GET /api/build-logs/id/{id}/stream; frames {stdout_append, stderr_append,
+// done, status}); the stream replays accumulated output on connect, so
+// attaching mid-build still shows the backlog.
+let liveLogEs = null;
+let liveLogId = null;
+let liveLogDone = false;
+let liveLogCollapsed = false;
+
+function closeLiveLogStream() {
+  if (liveLogEs) { liveLogEs.close(); liveLogEs = null; }
+}
+
+function renderRebuildLiveLog(queue) {
+  const root = $('rebuild-live-log');
+  if (!root) return;
+  const running = (queue || []).find(
+    (e) => e.state === 'running' && e.build_log_id != null);
+
+  if (!running) {
+    // No running build with a log → tear down + hide.
+    closeLiveLogStream();
+    liveLogId = null;
+    liveLogDone = false;
+    if (!root.hidden) { root.hidden = true; root.replaceChildren(); }
+    return;
+  }
+  // Same build still streaming (or already finished) → leave the panel as-is.
+  // Re-renders fire on every queue mutation; don't reconnect mid-build, and
+  // don't reopen a stream that already sent `done`.
+  if (running.build_log_id === liveLogId && (liveLogEs || liveLogDone)) return;
+
+  // New running build → (re)build the panel + open its stream.
+  closeLiveLogStream();
+  liveLogId = running.build_log_id;
+  liveLogDone = false;
+  root.hidden = false;
+  root.replaceChildren();
+
+  const pre = el('pre', { class: 'rebuild-live-log-output' }, '');
+  pre.hidden = liveLogCollapsed;
+  const badge = el('span', { class: 'rebuild-live-log-badge rll-running' }, 'live');
+  const toggle = el('button', {
+    type: 'button',
+    class: 'rebuild-live-log-toggle',
+    'aria-expanded': String(!liveLogCollapsed),
+    title: liveLogCollapsed ? 'expand live log' : 'collapse live log',
+  }, liveLogCollapsed ? '▸' : '▾');
+  toggle.addEventListener('click', () => {
+    liveLogCollapsed = !liveLogCollapsed;
+    pre.hidden = liveLogCollapsed;
+    toggle.textContent = liveLogCollapsed ? '▸' : '▾';
+    toggle.setAttribute('aria-expanded', String(!liveLogCollapsed));
+    toggle.title = liveLogCollapsed ? 'expand live log' : 'collapse live log';
+  });
+  const header = el('div', { class: 'rebuild-live-log-header' },
+    toggle, ' ',
+    el('span', { class: 'rebuild-live-log-title' }, 'live build log — '),
+    el('code', { class: 'rqe-agent' }, running.agent),
+    ' ', el('span', { class: 'rqe-kind' }, running.kind || 'rebuild'),
+    ' ', badge, ' ',
+    el('a', {
+      class: 'rebuild-live-log-raw',
+      href: '/api/build-logs/id/' + running.build_log_id + '/raw',
+      download: 'build-log-' + running.build_log_id + '.txt',
+    }, '↓ raw'),
+  );
+  root.append(header, pre);
+
+  // Sticky-bottom: auto-scroll to bottom on new lines unless the operator
+  // scrolled up (track intent via the scroll event — same as the logs page).
+  let atBottom = true;
+  pre.addEventListener('scroll', () => {
+    atBottom = pre.scrollHeight - pre.scrollTop - pre.clientHeight < 40;
+  });
+
+  let stderrSeen = false;
+  const es = new EventSource('/api/build-logs/id/' + running.build_log_id + '/stream');
+  liveLogEs = es;
+  es.onmessage = (ev) => {
+    let frame;
+    try { frame = JSON.parse(ev.data); } catch { return; }
+    if (frame.stdout_append) {
+      pre.textContent += frame.stdout_append;
+      if (atBottom) pre.scrollTop = pre.scrollHeight;
+    }
+    if (frame.stderr_append) {
+      if (!stderrSeen) { pre.textContent += '\n--- stderr ---\n'; stderrSeen = true; }
+      pre.textContent += frame.stderr_append;
+      if (atBottom) pre.scrollTop = pre.scrollHeight;
+    }
+    if (frame.done) {
+      liveLogDone = true;
+      badge.className = 'rebuild-live-log-badge '
+        + (frame.status === 'ok' ? 'rll-ok' : 'rll-fail');
+      badge.textContent = frame.status || 'done';
+      if (es === liveLogEs) { es.close(); liveLogEs = null; }
+      // Keep liveLogId + liveLogDone so a re-render before the entry leaves
+      // 'running' doesn't reopen the finished stream; the next render with no
+      // running entry clears the panel.
+    }
+  };
+  es.onerror = () => {
+    if (es.readyState === EventSource.CONNECTING) return; // transient reconnect
+    badge.className = 'rebuild-live-log-badge rll-fail';
+    badge.textContent = 'stream error';
+    if (es === liveLogEs) { es.close(); liveLogEs = null; }
+  };
+}
+
 // ─── kept state (tombstones) ──────────────────────────────────────────────
 function renderTombstones(s) {
   const root = $('tombstones-section');