Run create_thumbnail for both custom and extracted thumbnails

Modified gen_previews() to always call create_thumbnail() regardless of
whether a custom thumbnail exists or not. This ensures all thumbnails
(custom or extracted) are properly sized and formatted.

Previously, custom thumbnails would skip the create_thumbnail step,
potentially resulting in stretched or improperly sized thumbnails.

Addresses feedback from @jessegrabowski

Author: Copilot

scripts/generate_gallery.py

@@ -123,18 +123,17 @@ 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:
-            logger.warning(
-                f"Didn't find any pictures in {self.basename}",
-                type="thumbnail_extractor",
-            )
-            shutil.copy(self.default_image_loc, self.png_path)
+            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)
         create_thumbnail(self.png_path)