Phase 4: backups w/ volumes, notifications, settings & users, audit page (0.4.0)

- Backup/restore: per-stack tar.gz incl. named-volume snapshots (helper
  container), upload restore with rename/overwrite/conflict detection.
- Notifications: ntfy/Discord/Slack/Gotify/generic webhooks, per-event
  subscriptions; wired into the update checker and stack lifecycle.
- Settings page: update-check interval, webhook CRUD + test, user management
  (with last-admin safeguards).
- Audit log page (searchable, paginated).
- Mobile-responsive sidebar/layout.

Multi-host agents and remote backup destinations (SFTP/S3) deferred.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.1.0",
+  "version": "0.4.0",
   "type": "module",
   "scripts": {
     "dev": "vite",