feat(desktop): ask once what closing the window should do (cinny-desktop #5)

The first time the window is closed, a dialog asks "Keep Lotus Chat
running?" — Keep running in the tray (default, focused) / Quit when I
close the window — with an opt-in "Start Lotus Chat when I sign in"
checkbox in the same moment (per the approved design: one dialog, no
wizard). The choice is saved natively; Settings → General → "When I
close the window" changes it later (tray / quit / ask me).

The dialog is role="dialog" aria-modal, labelled and described, with
focus on the default button. Web-side flow verified with a stubbed
native side: event → dialog → checkbox + Quit sends autostart enable +
resolve_close_request("quit"); the Settings select saves "tray".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/pages/client/ClientNonUIFeatures.tsx

@@ -70,6 +70,7 @@ import { getRoomRetentionMs, isExpired } from '../../utils/retention';
 import { useTauriUpdateProgress, useTauriUpdater } from '../../hooks/useTauriUpdater';
 import { isNetworkUpdateError } from '../../utils/updateErrors';
 import { invokeTauri, isTauri as isTauriApp, useTauriEvent } from '../../hooks/useTauri';
+import { CloseBehaviorPrompt } from '../../components/CloseBehaviorPrompt';
 import { TauriDesktopFeatures } from '../../components/TauriDesktopFeatures';
 import { KeyboardShortcutsDialog, useKeyboardShortcutsTrigger } from '../../features/shortcuts';
 import { useRoomsListener } from '../../hooks/useRoomsListener';
@@ -1088,6 +1089,7 @@ export function ClientNonUIFeatures({ children }: ClientNonUIFeaturesProps) {
       <RetentionSweeper />
       <TauriUpdateFeature />
       <TauriDesktopFeatures />
+      <CloseBehaviorPrompt />
       <LotusDenoiseFeature />
       <DeepLinkNavigator />
       <KeyboardShortcutsFeature />