Fix low-severity robustness issues

- show_error_dialog() aborted when QApplication construction itself was
  the failure; fall back to stderr so the real error is visible.
- Auto-update QThread cleanup dropped the last Python reference while
  run() could still be unwinding ("QThread: Destroyed while thread is
  still running"); defer destruction to deleteLater on finished.
- macOS ffmpeg install no longer curl|bash-es the Homebrew bootstrap
  script unattended; it now asks the user to install Homebrew
  themselves and fails cleanly.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ytsage/main.py

@@ -7,6 +7,12 @@ from .gui.ytsage_gui_main import YTSageApp  # Import the main application class
 
 
 def show_error_dialog(message):
+    # A QMessageBox needs a live QApplication; if startup failed before (or
+    # while) creating one, constructing the dialog would abort the process
+    # and swallow the real error.
+    if QApplication.instance() is None:
+        print(f"Application Error: {message}", file=sys.stderr)
+        return
     error_dialog = QMessageBox()
     error_dialog.setIcon(QMessageBox.Icon.Critical)
     error_dialog.setText("Application Error")