make the jobs back button a link, delete the backbutton nobody used

The audit said adopt the shared BackButton. It is not shared: zero importers
since the initial commit, no barrel entry, and a label-plus-underline shape
that fits none of the icon-only back controls here. Adopting it would have
redesigned the Jobs header under cover of a navigation fix.

So: a Link, matching what ScriptJobDetail and DownloadJobDetail already do,
and the dead component goes. useNavigate had no other caller in
PipelineJobDetail and goes with it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/navigation-audit.md

@@ -234,7 +234,12 @@ publishers means changing the chat panel, which is another agent's, so it is wri
 ### Phase 4 — Polish + borderline decisions
 - [x] Dock + Header + mobile sheet → react-router `<NavLink>`; both `isActive` helpers and their `useLocation`s deleted. `end` on Home only. **Needs runtime test.**
 - [ ] "New Chat" → `<Link to="/chat/new">` (`SessionList.tsx:68`) once H4's channel cleanup lands.
-- [ ] Jobs back button + any `navigate('/jobs')` → shared `BackButton` (`JobDetail.tsx:709`).
+- [x] Jobs back button → `<Link to="/jobs">`, and `useNavigate` dropped from `PipelineJobDetail` (it had no
+      other caller). **Not** the shared `BackButton`, which turned out to be the second dead component this
+      audit has found: zero importers since the initial commit, no barrel entry, and a label-plus-underline
+      shape that fits none of the icon-only back controls in the app. Adopting it would have been a visual
+      redesign of the Jobs header dressed up as a navigation fix, so it is deleted instead. The two sibling
+      panes (`ScriptJobDetail`, `DownloadJobDetail`) were already `<Link to="/jobs">`; this was the odd one out.
 - [ ] Decide/skip: Jobs step deep-link, Preview slug, FileBrowser widget. (Browser tabs: **done** — see the LOW section. Monitor scope: **done** as M10, it was not a view toggle. Music favorites: **decided** — stays a channel, reasoning in the LOW section.)
 
 ### Cross-cutting for the refactor itself