fix(macos): retain MainWindowController so the client stays alive

The main window controller was created as a local variable in
ConnectWindowController.authSucceeded and never retained — ARC
deallocated it immediately, which destroyed the VoiceCatClient
(connection silently dropped), nil'd every button's weak target
(clicks did nothing), and killed event delivery (channel list,
messages, voice never worked). Symptom: TOFU (in the retained
connect controller) worked, but everything in the main window
was a zombie shell.

Fix: store the MainWindowController in a new field on
ConnectWindowController (which AppDelegate retains for the app's
lifetime). Also add NSLog diagnostics in deinit/bootstrap/handleEvent
so lifecycle and event delivery are observable from the terminal
or Console.app.

clients/apple/macOS/VoiceCatMac/Windows/ConnectWindowController.swift

@@ -18,6 +18,7 @@ final class ConnectWindowController: NSWindowController, NSWindowDelegate {
     private var servers: [SavedServer] = ServerListStore.load()
     private var client: VoiceCatClient?
     private var identityDialogShown = false
+    private var mainWindowController: MainWindowController?
 
     // MARK: - Init
 
@@ -298,6 +299,7 @@ final class ConnectWindowController: NSWindowController, NSWindowDelegate {
     private func authSucceeded(client: VoiceCatClient, selfUserId: UInt32, nickname: String) {
         client.onEvent = nil
         let mainWC = MainWindowController(client: client, selfUserId: selfUserId, nickname: nickname)
+        self.mainWindowController = mainWC
         mainWC.showWindow(nil)
         self.client = nil
         close()