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/core/ytsage_ffmpeg.py

@@ -406,9 +406,14 @@ def install_ffmpeg_macos() -> bool:
                 timeout=5,
             )
         except (subprocess.SubprocessError, FileNotFoundError):
-            logger.info("Installing Homebrew...")
-            brew_install_cmd = '/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"'
-            subprocess.run(brew_install_cmd, shell=True, check=True, timeout=300)
+            # Never curl|bash a remote script without the user's say-so.
+            # Homebrew installation is the user's decision to make in a
+            # terminal, where the script can also prompt for sudo properly.
+            logger.error(
+                "Homebrew is not installed. Install it from https://brew.sh and retry, "
+                "or install ffmpeg another way."
+            )
+            return False
 
         # Install FFmpeg
         logger.info("Installing FFmpeg...")