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.
50986b5a05