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

Author: Copilot

scripts/generate_gallery.py

@@ -118,6 +118,13 @@ def extract_preview_pic(self):
         return None
 
     def gen_previews(self):
+        if self.png_path.exists():
+            logger.info(
+                f"Custom thumbnail already exists for {self.basename}, skipping extraction",
+                type="thumbnail_extractor",
+            )
+            return
+
         preview = self.extract_preview_pic()
         if preview is not None:
             with self.png_path.open("wb") as buff: