reorder items in playlist

[onmywrist]

I asked my favourite AI assistant to add functionality to reorder items in a playlist.
It updated the following 3 files and the changes seem to work. I did not do any extensive testing.

  - model.py — added reorder_plugins(ordered_names) method to the Playlist class
  - playlist.py — added POST /reorder_plugins/<playlist_name> API endpoint
  - playlist.html — added a ≡ drag handle on each plugin row; dragging a row saves the new order automatically to
  device.json via the API (no page reload needed)

  Visit http://inkypi.local/playlist — you should see the hamburger handle on each plugin row. Just drag to reorder.

[playlist.html](https://github.com/user-attachments/files/26723377/playlist.html) [playlist.py](https://github.com/user-attachments/files/26723378/playlist.py) [model.py](https://github.com/user-attachments/files/26723379/model.py)