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...")

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")
 

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")