fix: stash playlist scroll before video load, restore after

Video load triggers layout changes (buttons, crop bar, preview window)
that cause Qt to recalculate the playlist scrollbar position. Now saves
the scroll value in _load_file and restores it at the end of _after_load.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

main.py

@@ -2213,6 +2213,10 @@ class MainWindow(QMainWindow):
         self._lbl_file.setText(os.path.basename(path))
         self.setWindowTitle(f"8-cut — {os.path.basename(path)}")
         _log(f"Loading: {os.path.basename(path)}")
+        # Stash playlist scroll — video load triggers layout changes that
+        # cause Qt to recalculate the scrollbar.
+        sb = self._playlist.verticalScrollBar()
+        self._playlist_scroll_stash = sb.value() if sb else 0
         self._mpv.load(path)
         # _after_load triggered by MpvWidget.file_loaded signal
 
@@ -2242,6 +2246,10 @@ class MainWindow(QMainWindow):
         self._spn_spread.setValue(float(self._settings.value("spread", "3.0")))
         self._preview_win.show()
         self._preview_timer.start()
+        # Restore playlist scroll that video load disturbed.
+        sb = self._playlist.verticalScrollBar()
+        if sb and hasattr(self, '_playlist_scroll_stash'):
+            sb.setValue(self._playlist_scroll_stash)
 
         # Run DB fuzzy match off the main thread — can be slow on large databases.
         filename = os.path.basename(self._file_path)