feat: Add cover art support for user client

This commit adds support for displaying cover art from the currently playing media in the Home Assistant user client.

- The `HassUserClient` now publishes cover art to the `user/image/cover` MQTT topic.
- The `publish_cover` method uses `playerctl` to retrieve the art URL and `pillow` to convert the image to WebP format before publishing.
- The `components` property now includes an `image` component for cover art.
- The `cover` attribute on `HassUserClient` is used to avoid resending the same cover multiple times.
- The `publish_state` method has been extended to spawn a thread for updating the cover.

.pre-commit-config.yaml

@@ -20,4 +20,4 @@ repos:
     hooks:
       - id: mypy
         args: [--strict]
-        additional_dependencies: [paho-mqtt]
+        additional_dependencies: [paho-mqtt, pillow]