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/Stacks.tsx

@@ -4,6 +4,7 @@ import { useQuery } from "@tanstack/react-query";
 import { Plus, Search } from "lucide-react";
 import { Button, Input, Spinner, Card } from "@/components/ui";
 import { StackCard } from "@/components/stacks/StackCard";
+import { RestoreButton } from "@/components/stacks/BackupRestore";
 import { stacksApi } from "@/api/stacks";
 import { useAuthStore } from "@/store/auth";
 import { useStackActions } from "@/hooks/useStackActions";
@@ -57,6 +58,7 @@ export function Stacks() {
           <option value="status">Sort: Status</option>
           <option value="updated">Sort: Last updated</option>
         </select>
+        {isAdmin && <RestoreButton />}
         {isAdmin && (
           <Link to="/stacks/new">
             <Button>