feat: add native player storyboard preview option

- Introduced `native_player_storyboard` setting with default disabled.
- Updated templates to conditionally load the storyboard preview script only for native players.
- Added descriptive comments for the new setting and template blocks.

youtube/templates/watch.html

@@ -304,8 +304,8 @@
         <!-- /plyr -->
     {% endif %}
 
-    <!-- Storyboard Preview Thumbnails -->
-    {% if settings.use_video_player != 2 %}
+    <!-- Storyboard Preview Thumbnails (native players only; Plyr handles this internally) -->
+    {% if settings.use_video_player != 2 and settings.native_player_storyboard %}
       <script src="/youtube.com/static/js/storyboard-preview.js"></script>
     {% endif %}