dashboard: render step sub-line on rebuild queue entries (closes #437)

Frontend half of #437 — paired with damocles's #501 backend which
adds `step: Option<String>` to `QueueEntry` and annotates 9
worker phases across ApplyCommit / Spawn / MetaUpdate / Rebuild
pipelines.

When `entry.step` is present, render a cyan `↳ <step>` sub-line
below the main row in `renderQueueEntry`. Wraps to its own flex
line via `flex-basis: 100%`, indented 1.8em to align under the
state glyph + kind. Terminal transitions clear `step` on the
backend, so the sub-line just disappears on Done / Failed rows —
no client-side staleness handling needed.

CSS: `.rqe-step` keys off cyan to visually group with the
running-spinner color cue (vs the muted `.rqe-when` /
`.rqe-reason` chips). `tabular-nums` keeps the indented arrow
column stable as step strings change length mid-build.

docs/web-ui.md updated to describe the new sub-line behaviour
in the R3BU1LD QU3U3 section.

frontend/packages/dashboard/src/tabs.js

@@ -1823,6 +1823,14 @@ window.marked = marked;
       const r = entry.reason.split('\n')[0];
       li.append(' ', el('span', { class: 'rqe-reason', title: entry.reason }, '— ' + truncate(r, 60)));
     }
+    // Current step (#437 / #501): backend annotates the in-flight
+    // phase on `running` entries — sub-line below the main row so the
+    // operator can see "what's happening right now" inside a long
+    // build. Terminal transitions clear `step` on the backend so this
+    // doesn't render stale labels on Done / Failed rows.
+    if (entry.step) {
+      li.append(el('div', { class: 'rqe-step' }, '↳ ' + entry.step));
+    }
     // Error block, when failed.
     if (entry.error) {
       li.append(el('pre', { class: 'rqe-error', title: entry.error }, truncate(entry.error, 200)));