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/App.tsx

@@ -8,7 +8,9 @@ import { StackDetail } from "@/pages/StackDetail";
 import { StackEditor } from "@/pages/StackEditor";
 import { Images } from "@/pages/Images";
 import { Templates } from "@/pages/Templates";
-import { Networks, Settings } from "@/pages/Placeholder";
+import { Settings } from "@/pages/Settings";
+import { Audit } from "@/pages/Audit";
+import { Networks } from "@/pages/Placeholder";
 import { useAuthStore } from "@/store/auth";
 import { useThemeStore } from "@/store/theme";
 
@@ -44,6 +46,7 @@ export default function App() {
             <Route path="/networks" element={<Networks />} />
             <Route path="/images" element={<Images />} />
             <Route path="/templates" element={<Templates />} />
+            <Route path="/audit" element={<Audit />} />
             <Route path="/settings" element={<Settings />} />
           </Route>
         </Route>