Hide music libraries from app (Issue #20)

- Filter out artist-type libraries in libraries_screen.dart
- Add safety check in media_card.dart to prevent music playback attempts
- Filter music content from getOnDeck() and getRecentlyAdded() in plex_client.dart

Music libraries are now hidden throughout the app since music playback
is not yet supported. This focuses the app on video content (movies and TV shows).

lib/widgets/media_card.dart

@@ -35,6 +35,19 @@ class _MediaCardState extends State<MediaCard> {
 
     final itemType = widget.item.type.toLowerCase();
 
+    // Music content is not yet supported
+    if (itemType == 'artist' || itemType == 'album' || itemType == 'track') {
+      if (context.mounted) {
+        ScaffoldMessenger.of(context).showSnackBar(
+          const SnackBar(
+            content: Text('Music playback is not yet supported'),
+            duration: Duration(seconds: 2),
+          ),
+        );
+      }
+      return;
+    }
+
     // For episodes, start playback directly
     if (itemType == 'episode') {
       final result = await navigateToVideoPlayer(