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.
bvandeusen
96aa2407d9