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>

frontend/src/api/volumes.ts

@@ -1,5 +1,5 @@
 import api from "./client";
-import type { HostPathResult, VolumeInfo } from "@/types";
+import type { HostPathResult, PruneStatus, VolumeInfo } from "@/types";
 
 const base = "/api/volumes";
 
@@ -11,8 +11,14 @@ export const volumesApi = {
       .then((r) => r.data),
   remove: (name: string, force = false) =>
     api.delete(`${base}/${name}?force=${force}`).then((r) => r.data),
-  prune: () =>
-    api.post<{ VolumesDeleted: string[] | null }>(`${base}/prune`).then((r) => r.data),
+  // Runs in the background on the server; poll pruneStatus() for progress.
+  startPrune: (allUnused: boolean) =>
+    api
+      .post<PruneStatus>(`${base}/prune`, null, {
+        params: { all: allUnused, background: true },
+      })
+      .then((r) => r.data),
+  pruneStatus: () => api.get<PruneStatus>(`${base}/prune-status`).then((r) => r.data),
   generateYaml: (spec: Record<string, unknown>) =>
     api
       .post<{ yaml: string }>("/api/volumes/generate-yaml", spec)