feat(backup): admin and account UI for server backups, import by section

Admin settings: switch, time, retention, list with download and a
"Jetzt sichern" button. Account settings: own server backups with
download and restore (replace or add, for chosen sections, replace needs
a second tap). The MindSift import gets the same section picker.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/settings/AdminSettingsView.tsx

@@ -28,6 +28,7 @@ import { api, setCurrentSharingMode, triggerHaptic } from '../../lib/api';
 import { passwordProblem } from '../../lib/password';
 import { UserAvatar } from '../../components/UserAvatar';
 import { ToggleSwitch } from '../../components/ToggleSwitch';
+import { BackupAdminCard } from './BackupAdminCard';
 import { CountBadge } from '../../components/CountBadge';
 
 interface AdminSettingsViewProps {
@@ -135,6 +136,9 @@ export const AdminSettingsView: React.FC<AdminSettingsViewProps> = ({
           smtpSkipVerify: Boolean(sys.smtpSkipVerify),
           allowPrivateWebhooks: Boolean(sys.allowPrivateWebhooks),
           smtpConfiguredViaEnv: Boolean(sys.smtpConfiguredViaEnv),
+          backupEnabled: Boolean(sys.backupEnabled),
+          backupTime: sys.backupTime || '03:00',
+          backupRetentionDays: sys.backupRetentionDays || 3,
         });
       }
     } catch (err: any) {
@@ -1093,6 +1097,8 @@ export const AdminSettingsView: React.FC<AdminSettingsViewProps> = ({
                   )}
                 </div>
 
+                <BackupAdminCard settings={settings} onSave={(patch) => handleSaveSystemSettings(patch)} />
+
                 {/* Alarm webhooks into the local network (SSRF protection) */}
                 <div className="p-4 bg-canvas border border-border rounded-card flex items-center justify-between gap-4">
                   <div className="pr-4">