feat(desktop): Settings security center (PIN change / recovery / reset)

Drops the manual backup-string flow; replaces it with PIN change,
recovery-code regeneration, and identity reset (all sealed via the new
user_keys table).

apps/desktop/src/components/AppShell.tsx

@@ -7,7 +7,6 @@ import { startDeviceApprovalListener } from '../lib/deviceApproval';
 import { ensureInstallId } from '../lib/installId';
 import { ensureNotificationPermission } from '../lib/osNotify';
 import { cachedUserKey } from '../lib/userIdentity';
-import { BackupPromptBanner } from './BackupPromptBanner';
 import { CallUI } from './CallUI';
 import { DeviceApprovalBanner } from './DeviceApprovalBanner';
 import { Sidebar } from './Sidebar';
@@ -53,7 +52,6 @@ export function AppShell() {
         </main>
       </div>
       <CallUI />
-      <BackupPromptBanner />
       <DeviceApprovalBanner />
     </div>
   );