* Initial plan

* Fix issue #1554: Preserve custom thumbnails in gallery script

Modified scripts/generate_gallery.py to check if a thumbnail file already
exists before extracting from notebooks. This prevents the script from
overwriting manually-placed custom thumbnails in the _thumbnails directory.

The gen_previews() method now:
1. Checks if self.png_path exists
2. If yes, logs info message and returns early (preserving custom thumbnail)
3. If no, proceeds with extraction from notebook or uses default logo

Fixes #1554

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>