polish: preview failure dialog + clean audition temp on close + drop redundant Qt imports

main.py

@@ -6659,16 +6659,16 @@ class MainWindow(QMainWindow):
             self._merge_list.takeItem(row)
 
     def _merge_paths(self) -> list[str]:
-        from PyQt6.QtCore import Qt
         return [self._merge_list.item(i).data(Qt.ItemDataRole.UserRole)
                 for i in range(self._merge_list.count())]
 
     def _merge_guard_ok(self, crossfade: float) -> bool:
         """False (+ status) if a clip with a KNOWN duration is <= the crossfade
         (acrossfade needs each clip longer than the crossfade)."""
-        from PyQt6.QtCore import Qt
         if crossfade <= 0:
             return True
+        # Clips whose duration couldn't be probed (None) are not flagged here;
+        # if genuinely too short they surface as an ffmpeg error at render.
         for i in range(self._merge_list.count()):
             it = self._merge_list.item(i)
             d = it.data(Qt.ItemDataRole.UserRole + 1)
@@ -6752,7 +6752,9 @@ class MainWindow(QMainWindow):
         if ok:
             self._play_file(tmp)
         else:
-            self._show_status(f"Preview failed: {err}" if err else "Preview render failed", 5000)
+            self._show_status("Preview failed", 4000)
+            QMessageBox.warning(self, "Preview failed",
+                                f"Could not render preview:\n\n{err}")
 
     def _on_wave_refresh(self) -> None:
         """Decode the current audio area and repaint the waveform strip."""
@@ -6797,6 +6799,7 @@ class MainWindow(QMainWindow):
         dur = self._spn_audio_len.value()
         edit_filters = self._current_edit_filters()
         tmp = os.path.join(tempfile.gettempdir(), "8cut_audition.wav")
+        self._merge_temps.add(tmp)
         cmd = build_audio_clip_command(self._file_path, start, dur, tmp,
                                        filters=edit_filters or None)
         QApplication.setOverrideCursor(Qt.CursorShape.WaitCursor)