Fix socket accept loop not restarted after Sparkle update relaunch

After a Sparkle auto-update relaunches cmux, the control socket stops
accepting connections because start() early-returns when isRunning is
true, without checking if the accept loop thread is actually alive.

- Add acceptLoopAlive flag to track accept loop thread liveness
- Fix start() early-return to also check acceptLoopAlive, so a dead
  thread triggers full socket re-creation
- Break acceptLoop() after 50 consecutive accept() failures with 10ms
  backoff instead of tight-spinning forever
- Clean up socket in applicationWillTerminate and
  updaterWillRelaunchApplication for clean teardown before relaunch

Sources/AppDelegate.swift

@@ -424,6 +424,7 @@ final class AppDelegate: NSObject, NSApplicationDelegate, UNUserNotificationCent
     }
 
     func applicationWillTerminate(_ notification: Notification) {
+        TerminalController.shared.stop()
         BrowserHistoryStore.shared.flushPendingSaves()
         PostHogAnalytics.shared.flush()
         notificationStore?.clearAll()