feat(updates): auto-prune dangling images after updates (#1316)

* feat(updates): auto-prune dangling images after updates

Each update pulls a fresh image and recreates containers, leaving the
replaced image behind as a dangling layer that previously had to be
pruned by hand. A new "Prune dangling images after updates" toggle under
Settings > System > Docker hygiene reclaims these automatically.

The setting is on by default and opt-out. When enabled, a successful
stack update (manual or scheduled) and a Sencho self-update each remove
the dangling image layers they orphaned. Only untagged layers are
touched; tagged images, volumes, and data are never removed. The toggle
requires an admin account and is per node: each instance honors its own
value, so a remote node self-update applies that node's own preference.

A prune failure never affects the update result: on the stack path it is
caught and logged after the update has already succeeded, and on the
self-update path the helper-shell prune runs only after a clean recreate
and cannot change the exit code or the recorded update error.

* security(self-update): shell-quote label-derived values in helper command

Address review feedback on the prune-on-update change:

- The self-update helper command interpolated the compose service name and
  config-file paths (both read from Docker Compose labels) straight into a
  shell string. Shell-quote them via shQuote so a label carrying shell
  metacharacters stays inert data and cannot break the exit-code capture,
  error-file write, or prune guard.
- Correct the settings copy and docs: the prune is a standard dangling-image
  prune, so it reclaims every untagged layer on the node, not only the one the
  current update orphaned. Tagged images, volumes, and data remain untouched.
- Add tests: shell-metacharacter neutralization and prune-output suppression in
  the self-update command, and an atomic-update case asserting a prune failure
  does not trigger a rollback.

* fix(updates): omit the reclaim figure when the daemon reports zero bytes

End-to-end testing on a Docker daemon backed by the containerd image store
showed the post-update prune removing a dangling image while the prune API
returned SpaceReclaimed=0, so the stream printed "reclaimed 0.0 MB" even though
an image was removed. Show the reclaimed figure only when the daemon reports a
non-zero value; otherwise the line reads "=== Pruned dangling images ===". The
overlay2 store still reports real figures and shows them. Add a test covering
both branches.

backend/src/services/DockerController.ts

@@ -263,6 +263,16 @@ class DockerController {
     };
   }
 
+  // Prune ONLY dangling (untagged) images. Distinct from pruneSystem('images'),
+  // which uses { dangling: { 'false': true } } to remove every unused image.
+  // Used by the prune-on-update flow to reclaim the layers a pull/recreate
+  // orphans, without touching tagged images for stopped stacks.
+  public async pruneDanglingImages(): Promise<{ success: boolean; reclaimedBytes: number }> {
+    const filters: Record<string, string[] | Record<string, boolean>> = { dangling: { 'true': true } };
+    const r = await this.docker.pruneImages({ filters });
+    return { success: true, reclaimedBytes: r.SpaceReclaimed || 0 };
+  }
+
   public async getImages() {
     const data = await this.docker.listImages({ all: false });
     return this.validateApiData<any[]>(data);