Show live progress while pruning volumes, and ask first (0.62.0)

The volume prune gets the same treatment as the image prune in 0.61.0:
the candidates are listed and removed one by one in the background, and
the Volumes page shows a progress bar (x / y, the volume being removed,
failures) followed by a toast with the space freed.

The button also used to delete without asking, and a bare docker volume
prune only removes anonymous volumes on Engine 23+ with no way to include
named ones. It now opens a confirmation dialog with an explicit -a option
that warns it also takes the data of every stack that is down.

- Candidates: volumes no container (running or stopped) mounts, local
  scope only, and anonymous (com.docker.volume.anonymous) unless -a.
  Volumes from pre-23 daemons carry no such label, so they only go with
  -a, which errs on the side of keeping data.
- Removed without force, so a volume mounted since the listing is
  refused; one refusal does not stop the rest.
- No closing daemon prune: nothing is left over, and on a pre-23 daemon an
  unfiltered one would delete named volumes too.
- Freed space is the sum of the deleted volumes' sizes (volumes share no
  data), from the size cache, which is invalidated afterwards.
- POST /api/volumes/prune takes all= and background=; without background
  it still waits and returns {VolumesDeleted, SpaceReclaimed}.
  GET /api/volumes/prune-status is admin-only.
- volume_sizes() ("Compute sizes") now uses the long-timeout client; it
  walks every volume and could hit the same 60s limit.

The progress machinery moved into services/prune_job.py (PruneJob,
PrunePlan) and, on the frontend, the usePruneJob hook and PruneProgress
component, shared by both pages. The status now carries a phase
(preparing / removing / finishing) so the parts without a count say what
they are doing.

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

backend/routers/images.py

@@ -11,6 +11,7 @@ from database import engine, get_session
 from models.image_scan import ImageScan, ScanDetail, ScanRequest, ScanSummary
 from models.user import User
 from services import audit_service, image_service, scan_service, update_service
+from services.prune_job import PruneBusy
 
 router = APIRouter(prefix="/api/images", tags=["images"])
 
@@ -69,7 +70,7 @@ def prune(
             response.status_code = 202
             return image_service.start_prune(all_unused, on_done=audit)
         result = image_service.prune_images(all_unused)
-    except image_service.PruneBusy as exc:
+    except PruneBusy as exc:
         raise HTTPException(status_code=409, detail=str(exc)) from exc
     audit(result)
     return result