feat(server): add EnableCoverArtUpload config option

Allow administrators to disable playlist cover art upload/removal for
non-admin users via the new EnableCoverArtUpload config option (default: true).

- Guard uploadPlaylistImage and deletePlaylistImage endpoints (403 for non-admin when disabled)
- Set CoverArtRole in Subsonic GetUser/GetUsers responses based on config and admin status
- Pass config to frontend and conditionally hide upload/remove UI controls
- Admins always retain upload capability regardless of setting

ui/src/playlist/PlaylistDetails.jsx

@@ -20,6 +20,7 @@ import {
   SizeField,
   isWritable,
 } from '../common'
+import config from '../config'
 import subsonic from '../subsonic'
 import { REST_URL } from '../consts'
 import { httpClient } from '../dataProvider'
@@ -134,7 +135,9 @@ const PlaylistDetails = (props) => {
 
   const imageUrl = subsonic.getCoverArtUrl(record, 300, true)
   const fullImageUrl = subsonic.getCoverArtUrl(record)
-  const canEdit = isWritable(record.ownerId)
+  const canEdit =
+    isWritable(record.ownerId) &&
+    (config.enableCoverArtUpload || localStorage.getItem('role') === 'admin')
 
   // Reset image state when playlist changes
   useEffect(() => {