feat(home): Songs-like → dedicated row + wider spread (#1491)

Promote the best-performing surface ("Songs like {artist}", ~8% skip /
~86% completion) out of the shared Playlists carousel into its own Home
row on both Android and web, and widen the daily build from 3 to 6 mixes
so the dedicated row shows a wider spread.

Server (internal/playlists):
- PickSeedArtists candidate pool 5 → 12; pickSeedArtistsForDay now takes
  songsLikeSeedCount (6) instead of a hardcoded 3. Graceful degradation
  and daily rotation preserved.

Android (HomeScreen.kt):
- New songsLikeSection + buildSongsLikeRow; PlaylistsRow takes a title so
  it renders both the "Playlists" and "Songs like…" rows. buildOnlineRow
  / orderedRealPlaylists no longer reserve the 3 songs-like slots.
  Offline shows cached mixes (available-first), hides the row when none.

Web (+page.svelte):
- Dedicated "Songs like…" row from songsLikeRow; dropped the 3-slot cap
  and removed songs-like from the Playlists carousel.

Tests: seed_selection_test.go, BuildPlaylistsRowTest.kt, page.test.ts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/playlists/system.go

@@ -35,7 +35,7 @@ type seedArtistRow struct {
 }
 
 // pickSeedArtistsFromRows projects sqlc rows into the seed list. The
-// SQL already orders by score DESC + artist_id and LIMIT 3, so this is
+// SQL already orders by score DESC + artist_id and LIMIT 12, so this is
 // just a column projection — but pulling it into a function keeps the
 // call-site readable and makes the post-fetch path testable without
 // a database.
@@ -110,10 +110,20 @@ func pickDailySeeds(pool []pgtype.UUID, userID pgtype.UUID, dateStr string, n in
 	return shuffled[:n]
 }
 
-// pickSeedArtistsForDay picks up to 3 seed artists for the Songs-like
-// mixes; For-You uses pickDailySeeds directly with forYouSeedCount.
+// songsLikeSeedCount is how many "Songs like {artist}" mixes to build
+// each day (#1491). Bumped from 3 when Songs-like was promoted to its
+// own dedicated Home row — the best-performing surface (low skip / high
+// completion) was buried as 3 tiles in the shared Playlists carousel, so
+// the wider row now shows a wider spread. Drawn from PickSeedArtists'
+// deeper top-12 pool via a daily-deterministic shuffle so the set still
+// rotates day to day rather than pinning the same six artists.
+const songsLikeSeedCount = 6
+
+// pickSeedArtistsForDay picks up to songsLikeSeedCount seed artists for
+// the Songs-like mixes; For-You uses pickDailySeeds directly with
+// forYouSeedCount.
 func pickSeedArtistsForDay(pool []pgtype.UUID, userID pgtype.UUID, dateStr string) []pgtype.UUID {
-	return pickDailySeeds(pool, userID, dateStr, 3)
+	return pickDailySeeds(pool, userID, dateStr, songsLikeSeedCount)
 }
 
 // rankedCandidate is a (track_id, score) pair used during in-memory
@@ -589,8 +599,8 @@ func produceForYou(
 	return []builtPlaylist{{Name: "For You", Variant: "for_you", Tracks: tracks}}, nil
 }
 
-// produceSeedMixes: up to 3 "Songs like {artist}" mixes. Seed
-// artists rotate daily-deterministically; the seed query falls back
+// produceSeedMixes: up to songsLikeSeedCount "Songs like {artist}"
+// mixes. Seed artists rotate daily-deterministically; the seed query falls back
 // through widening engagement windows (#1255) and every returned row
 // shares the winning tier, stamped onto the built tracks as their
 // pick_kind. The base seed-artist query failing is fatal; per-artist
@@ -685,7 +695,7 @@ func produceDiscover(
 }
 
 // BuildSystemPlaylists builds the user's daily system mixes (one For-You +
-// up to 3 Songs-like-{seed} mixes). Atomic-replace inside one tx;
+// up to songsLikeSeedCount Songs-like-{seed} mixes). Atomic-replace inside one tx;
 // concurrency-guarded via system_playlist_runs.in_flight; deterministic
 // within a day via tieBreakHash(track_id, now.UTC().Format("2006-01-02")).
 //