Reset shared HTTP clients on startup to prevent event loop closed error

Cacheless mode closes the app while clients may still reference
the old event loop. Resetting to None on startup forces fresh
client creation on the new event loop.

booru_viewer/gui/app.py

@@ -227,6 +227,13 @@ class BooruApp(QMainWindow):
         self._async_thread = threading.Thread(target=self._async_loop.run_forever, daemon=True)
         self._async_thread.start()
 
+        # Reset shared HTTP clients from previous session
+        from ..core.cache import _get_shared_client
+        from ..core.api.base import BooruClient
+        BooruClient._shared_client = None
+        import booru_viewer.core.cache as _cache_mod
+        _cache_mod._shared_client = None
+
         self._setup_signals()
         self._setup_ui()
         self._setup_menu()