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/routes/settings.ts

@@ -24,6 +24,7 @@ const ALLOWED_SETTING_KEYS = new Set([
   'audit_retention_days',
   'mesh_auto_recreate',
   'scan_history_per_image_limit',
+  'prune_on_update',
 ]);
 
 // Keys whose write requires a paid license, not just an admin role.
@@ -46,6 +47,7 @@ const SettingsPatchSchema = z.object({
   audit_retention_days: z.coerce.number().int().min(1).max(365).transform(String),
   mesh_auto_recreate: z.enum(['0', '1']),
   scan_history_per_image_limit: z.coerce.number().int().min(5).max(1000).transform(String),
+  prune_on_update: z.enum(['0', '1']),
 }).partial();
 
 export const settingsRouter = Router();