fix(ansible): surface run failure reason; widget audit cleanup

Run UX: when start_run throws before/around launch (e.g. ENOSPC creating the
temp dir — the box is out of disk), the run was marked "failed" with empty
output. Now the exception is broadcast + written to the run output/results so
the run view shows e.g. "[run error] OSError: [Errno 28] No space left on
device" instead of a blank failure.

Widget audit follow-ups (no broken links were found; these are consistency):
- host_resource_history widget now charts root (/) disk, consistent with the
  host panel (was the opaque "disk worst").
- host_resources widget: tooltip on the health dot explaining it warns on the
  worst mount while the number shows root.
- status_overview widget detail_url /status → /status/ (avoid redirect).
- Normalize ad-hoc widget empty-states to the shared .empty style (wording,
  which distinguishes "configured" vs "data yet", preserved).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

steward/ansible/executor.py

@@ -462,9 +462,16 @@ async def start_run(
 
     except _Aborted:
         final_status = "cancelled"
-    except Exception:
+    except Exception as exc:
         logger.exception("Ansible run %s failed with exception", run_id)
         final_status = "cancelled" if run_id in _cancelled else "failed"
+        # Surface WHY in the run output instead of an empty "failed" (e.g. ENOSPC
+        # when the temp dir can't be created — the run never reaches Ansible).
+        msg = f"[run error] {type(exc).__name__}: {exc}"
+        _broadcast(run_id, msg)
+        if len(failures) < _FAILURE_CAP:
+            failures.append(msg[:500])
+        db_output = (db_output + "\n" + msg) if db_output else msg
     finally:
         if logf is not None:
             logf.close()