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/tests/test_docker_timeouts.py

@@ -37,12 +37,12 @@ def test_safe_call_wraps_connection_error():
 def test_volume_prune_timeout_is_a_502_with_detail(as_admin, monkeypatch):
     from services import volume_service
 
-    class Volumes:
-        def prune(self, filters=None):
+    class Api:
+        def containers(self, all=False):
             raise requests.exceptions.ReadTimeout("Read timed out.")
 
     class Client:
-        volumes = Volumes()
+        api = Api()
 
     monkeypatch.setattr(volume_service, "get_long_client", lambda: Client())
     r = as_admin.post("/api/volumes/prune")
@@ -53,7 +53,6 @@ def test_volume_prune_timeout_is_a_502_with_detail(as_admin, monkeypatch):
 @pytest.mark.parametrize(
     "module, fn, args",
     [
-        ("volume_service", "prune_volumes", ()),
         ("network_service", "prune_networks", ()),
     ],
 )
@@ -80,5 +79,6 @@ def test_prunes_use_the_long_timeout_client(monkeypatch, module, fn, args):
     assert used == ["long"]
 
 
-# The image prune goes image by image and has its own tests in
-# test_image_prune.py, including that it uses the long-timeout client.
+# The image and volume prunes go item by item and have their own tests in
+# test_image_prune.py / test_volume_prune.py, including that they use the
+# long-timeout client.