feat(ui): confirm main-window close when Mission Control sessions are open

Closing the main window used to silently kill every open Mission Control
ConPTY session (App's ShutdownMode.OnMainWindowClose tears the process down
without warning). Intercept Window.Closing, and when at least one pane is
open, ask via the existing ConfirmAsync pattern before proceeding; cancelling
leaves the window and sessions alive, confirming force-closes as before.

src/ClaudeDo.Localization/locales/de.json

@@ -714,6 +714,7 @@
   "vm": {
     "connection": { "online": "Online", "connecting": "Verbinden…", "offline": "Offline" },
     "shell": { "restartingWorker": "Worker wird neu gestartet…", "unexpectedError": "Unerwarteter Fehler: {0}" },
+    "mainWindow": { "closeConfirm": { "message": "{0} Mission-Control-Sitzung(en) laufen noch. Beim Beenden werden sie abgebrochen. Trotzdem schließen?" } },
     "agentStatus": { "idle": "Leerlauf", "queued": "In Warteschlange", "running": "Läuft", "review": "Prüfung", "children": "Wartet auf Teilaufgaben", "done": "Fertig", "failed": "Fehlgeschlagen", "cancelled": "Abgebrochen" },
     "taskStatus": { "idle": "Leerlauf", "queued": "In Warteschlange", "running": "Läuft", "waitingForReview": "Wartet auf Prüfung", "waitingForChildren": "Wartet auf Teilaufgaben", "done": "Fertig", "failed": "Fehlgeschlagen", "cancelled": "Abgebrochen", "parked": "Geparkt", "interactive": "Interaktiv" },
     "failureReason": { "maxTurns": "Turn-Limit erreicht", "timeout": "Zeitüberschreitung", "error": "Fehler", "cancelled": "Abgebrochen", "unknown": "Grund unbekannt" },