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.

docs/web-ui.md

@@ -208,9 +208,14 @@ timing, and an optional reason / error. Meta-update cascade
 rebuilds nest under their parent entry (`parent_id` grouping;
 `rqe-child` CSS class). Dedup: re-enqueueing a still-queued op
 for the same agent collapses into the existing entry. Running
-entries tick elapsed seconds live. Cold-loaded from
-`/api/state.rebuild_queue`; live updates via `rebuild_queue_changed`
-snapshot event.
+entries tick elapsed seconds live, and when the worker has
+annotated the current phase (#437) a cyan `↳ <step>` sub-line
+appears under the main row showing the in-flight step name
+(e.g. `↳ meta prepare_deploy` → `↳ nixos-container update` →
+`↳ finalize deploy`). Terminal transitions clear `step` on the
+backend so Done / Failed rows don't render stale labels.
+Cold-loaded from `/api/state.rebuild_queue`; live updates via
+`rebuild_queue_changed` snapshot event.
 
 **K3PT ST4T3** — destroyed-but-state-kept tombstones (size +
 age + claude-creds badge). Two actions: `⊕ R3V1V3` (queues a