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)

web/src/routes/+page.svelte

@@ -39,6 +39,10 @@
     (systemPlaylistsQ.data?.owned ?? []).find((p) => p.system_variant === 'for_you') ?? null
   );
 
+  const discoverPlaylist = $derived(
+    (systemPlaylistsQ.data?.owned ?? []).find((p) => p.system_variant === 'discover') ?? null
+  );
+
   const songsLikePlaylists = $derived(
     (systemPlaylistsQ.data?.owned ?? [])
       .filter((p) => p.system_variant === 'songs_like_artist')
@@ -48,7 +52,7 @@
   const userPlaylists = $derived(userPlaylistsQ.data?.owned ?? []);
 
   type PlaceholderVariant = 'building' | 'failed' | 'pending' | 'seed-needed';
-  function placeholderVariant(slot: 'for-you' | 'songs-like'): PlaceholderVariant {
+  function placeholderVariant(slot: 'for-you' | 'discover' | 'songs-like'): PlaceholderVariant {
     const s = systemStatusQ.data;
     if (!s) return 'pending';
     if (s.in_flight) return 'building';
@@ -71,7 +75,16 @@
       out.push({ kind: 'placeholder', label: 'For You', variant: placeholderVariant('for-you') });
     }
 
-    // Slots 2-4: Songs-like (real first, padded with placeholders).
+    // Slot 2: Discover (real or placeholder). Server emits this with
+    // system_variant='discover' once the recommendation engine has
+    // eligible tracks.
+    if (discoverPlaylist) {
+      out.push({ kind: 'real', playlist: discoverPlaylist });
+    } else {
+      out.push({ kind: 'placeholder', label: 'Discover', variant: placeholderVariant('discover') });
+    }
+
+    // Slots 3-5: Songs-like (real first, padded with placeholders).
     for (let i = 0; i < 3; i++) {
       if (songsLikePlaylists[i]) {
         out.push({ kind: 'real', playlist: songsLikePlaylists[i] });