feat(home): surface Discover system playlist as a tile (web + flutter)

Server has been generating system_variant='discover' playlists since
M6a (internal/playlists/system.go and POST
/api/playlists/system/discover/refresh) but neither client surfaced
it. The Flutter home filtered system playlists by exact match on
'for_you' and 'songs_like_artist', dropping Discover. The web home
did the same. Tiles were generated server-side and silently
discarded by both clients.

Add a Discover slot to the playlists row in both:
- Flutter: 5-slot row now (For You, Discover, 3× Songs-like) with
  matching placeholder state machine.
- Web: same shape, same placeholderVariant() call.

When the engine has built it, the real playlist tile renders;
otherwise a placeholder with the same building/pending/failed
status semantics as the other system tiles.

flutter_client/lib/library/home_screen.dart

@@ -135,7 +135,16 @@ List<_PlaylistRowItem> _buildPlaylistsRow(
       ? _RealPlaylist(forYou)
       : _PlaceholderPlaylist('For You', _variantFor('for-you', status)));
 
-  // Slots 2-4: Songs-like (real first, padded to 3).
+  // Slot 2: Discover. Server emits this with system_variant='discover'
+  // when the recommendation engine has eligible tracks; before then,
+  // show a placeholder in the same "building/pending" state machine
+  // as For-You.
+  final discover = findFirst((p) => p.systemVariant == 'discover');
+  out.add(discover != null
+      ? _RealPlaylist(discover)
+      : _PlaceholderPlaylist('Discover', _variantFor('discover', status)));
+
+  // Slots 3-5: Songs-like (real first, padded to 3).
   final songsLike = ownedAll
       .where((p) => p.systemVariant == 'songs_like_artist')
       .take(3)