GUI/CLI fixes: crash paths, privacy-lock bypass, threading, logout leaks

- _on_image_download_failed called self.statusBar() which does not
  exist on a QWidget -> AttributeError crash; use status_bar label.
- Ctrl+Shift+P no longer disables the privacy overlay while the session
  is password-locked (lock bypass).
- Registration code confirmation no longer touches Qt widgets from the
  asyncio thread; new AsyncBridge.confirm_result signal carries the
  result back to the Qt thread.
- MainWindow.closeEvent now disconnects all bridge signal connections
  (tracked in _bridge_connections), removes the theme listener and
  stops the periodic refresh timer — every logout/login cycle leaked a
  window that kept handling notifications (duplicate mark_read, tray
  toasts).
- AsyncBridge logout rewires _key_change_cb onto the fresh ChatClient
  (key-change MITM warning was dead after logout) and clears
  _pending_send_queue so queued messages cannot be sent under a
  different identity.
- CLI: fix await precedence crash in the react-to-message prompt.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client.py

@@ -348,7 +348,7 @@ async def interactive_menu(client: ChatClient):
                 print("[!] Invalid number.")
                 continue
             print("Reactions: thumbsup, heart, laugh, surprised, sad, thumbsdown")
-            reaction = await prompt("Reaction: ").strip().lower()
+            reaction = (await prompt("Reaction: ")).lower()
             if reaction not in ("thumbsup", "heart", "laugh", "surprised", "sad", "thumbsdown"):
                 print("[!] Invalid reaction.")
                 continue