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/ComposeService.ts

@@ -527,6 +527,24 @@ export class ComposeService {
       }
 
       sendOutput('=== Stack updated successfully ===\n');
+      // Opt-out (default ON): after a clean update, prune the node's dangling
+      // (untagged) image layers, including the one this pull just orphaned. Read
+      // fresh each run so a remote node honors its own setting. Wrapped so a
+      // prune failure can never reach the atomic-rollback catch below.
+      try {
+        const pruneOnUpdate = DatabaseService.getInstance().getGlobalSettings()['prune_on_update'] === '1';
+        if (pruneOnUpdate) {
+          const result = await DockerController.getInstance(this.nodeId).pruneDanglingImages();
+          // The Docker prune API does not report SpaceReclaimed on the containerd
+          // image store, so only show the figure when the daemon actually returns one.
+          const reclaimed = result.reclaimedBytes > 0
+            ? ` · reclaimed ${(result.reclaimedBytes / (1024 * 1024)).toFixed(1)} MB`
+            : '';
+          sendOutput(`=== Pruned dangling images${reclaimed} ===\n`);
+        }
+      } catch (pruneError) {
+        console.warn('Failed to prune dangling images after update for %s:', sanitizeForLog(stackName), pruneError);
+      }
       if (debug) console.debug(`[ComposeService:debug] updateStack completed in ${Date.now() - t0}ms`, { stackName });
     } catch (updateError) {
       if (atomic) {