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/gui/ytsage_gui_main.py

@@ -1176,12 +1176,16 @@ class YTSageApp(QMainWindow, FormatTableMixin, VideoInfoMixin, AnalysisMixin):
 
         # Clean up the thread reference and ensure it's properly finished
         if hasattr(self, "auto_update_thread"):
+            thread = self.auto_update_thread
             # Disconnect all signals to prevent further callbacks
-            self.auto_update_thread.update_finished.disconnect()
-            # Make sure thread is finished
-            if self.auto_update_thread.isRunning():
-                self.auto_update_thread.quit()
-                self.auto_update_thread.wait(1000)  # Wait up to 1 second
+            thread.update_finished.disconnect()
+            # Dropping the Python reference while run() is still unwinding
+            # can destroy a live QThread; let Qt delete it once finished.
+            if thread.isRunning():
+                thread.finished.connect(thread.deleteLater)
+                thread.quit()
+            else:
+                thread.deleteLater()
             # Remove the reference
             delattr(self, "auto_update_thread")