Revert: Do not run create_thumbnail for custom thumbnails

Reverted the change that made create_thumbnail run for custom thumbnails.
Custom thumbnails were losing resolution and quality when processed.

When a custom thumbnail exists, the script now:
- Logs that a custom thumbnail was found
- Returns early without processing it
- Preserves the original custom thumbnail as-is

This addresses feedback that processing custom thumbnails resulted in
worse quality and resolution.

Reverts commit c07e749

Author: Copilot

scripts/generate_gallery.py

@@ -123,17 +123,18 @@ def gen_previews(self):
                 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:
+                buff.write(preview)
         else:
-            preview = self.extract_preview_pic()
-            if preview is not None:
-                with self.png_path.open("wb") as buff:
-                    buff.write(preview)
-            else:
-                logger.warning(
-                    f"Didn't find any pictures in {self.basename}",
-                    type="thumbnail_extractor",
-                )
-                shutil.copy(self.default_image_loc, self.png_path)
+            logger.warning(
+                f"Didn't find any pictures in {self.basename}",
+                type="thumbnail_extractor",
+            )
+            shutil.copy(self.default_image_loc, self.png_path)
         create_thumbnail(self.png_path)