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.

PKGBUILD

@@ -15,7 +15,7 @@ arch=(any)
 url="https://git.edgarpierre.fr/edpibu/hasspy"
 license=('GPL-3.0-or-later')
 groups=()
-depends=('python-paho-mqtt')
+depends=('python-paho-mqtt' 'python-pillow')
 makedepends=('git' 'uv' 'python-installer') # 'bzr', 'git', 'mercurial' or 'subversion'
 provides=("${pkgname%-git}")
 conflicts=("${pkgname%-git}")