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/en.json

@@ -714,6 +714,7 @@
   "vm": {
     "connection": { "online": "Online", "connecting": "Connecting…", "offline": "Offline" },
     "shell": { "restartingWorker": "Restarting worker…", "unexpectedError": "Unexpected error: {0}" },
+    "mainWindow": { "closeConfirm": { "message": "{0} Mission Control session(s) are still running. Closing the app will end them. Close anyway?" } },
     "agentStatus": { "idle": "Idle", "queued": "Queued", "running": "Running", "review": "Review", "children": "Waiting for Subtasks", "done": "Done", "failed": "Failed", "cancelled": "Cancelled" },
     "taskStatus": { "idle": "Idle", "queued": "Queued", "running": "Running", "waitingForReview": "Waiting for Review", "waitingForChildren": "Waiting for Subtasks", "done": "Done", "failed": "Failed", "cancelled": "Cancelled", "parked": "Parked", "interactive": "Interactive" },
     "failureReason": { "maxTurns": "Turn limit reached", "timeout": "Timed out", "error": "Error", "cancelled": "Cancelled", "unknown": "Unknown reason" },