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/src/pages/StackDetail.tsx

@@ -12,6 +12,7 @@ import {
 } from "lucide-react";
 import { Badge, Button, Card, Spinner, StatusDot } from "@/components/ui";
 import { LogViewer } from "@/components/stacks/LogViewer";
+import { BackupButton } from "@/components/stacks/BackupRestore";
 import { stacksApi } from "@/api/stacks";
 import { useAuthStore } from "@/store/auth";
 import { useStackActions } from "@/hooks/useStackActions";
@@ -67,6 +68,7 @@ export function StackDetail() {
             <Button variant="outline" onClick={() => actions.down(id)} loading={busy}>
               <Power className="h-4 w-4" /> Down
             </Button>
+            <BackupButton stackId={id} />
             <Link to={`/stacks/${id}/edit`}>
               <Button>
                 <Pencil className="h-4 w-4" /> Edit