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>

web/src/routes/page.test.ts

@@ -1,5 +1,5 @@
 import { afterEach, beforeEach, describe, expect, test, vi } from 'vitest';
-import { render, screen } from '@testing-library/svelte';
+import { render, screen, within } from '@testing-library/svelte';
 import { readable } from 'svelte/store';
 import { emptyLikesMock } from '../test-utils/mocks/likes';
 import type { Playlist } from '$lib/api/types';
@@ -57,7 +57,8 @@ afterEach(() => vi.clearAllMocks());
 
 describe('home Playlists section', () => {
   test('renders 5 placeholder cards when no playlists exist', () => {
-    // Slot layout: For-You, Discover, 3× Songs-like.
+    // For-You + Discover in the Playlists row, plus 3 in the dedicated
+    // Songs-like row (#1491) = 5 placeholders total.
     render(Page);
     const placeholders = screen.queryAllByTestId('playlist-placeholder-card');
     expect(placeholders).toHaveLength(5);
@@ -107,12 +108,52 @@ describe('home Playlists section', () => {
     render(Page);
     expect(screen.getByText('For You')).toBeInTheDocument();
     const placeholders = screen.queryAllByTestId('playlist-placeholder-card');
-    // 1 Discover + 3 Songs-like slots = 4 placeholders alongside the
-    // real For-You tile.
+    // 1 Discover placeholder (Playlists row) + 3 Songs-like placeholders
+    // (dedicated row) = 4 alongside the real For-You tile.
     expect(placeholders).toHaveLength(4);
   });
 
-  test('renders secondary system kinds (deep_cuts / new_for_you) after Songs-like slots', () => {
+  test('dedicated Songs-like row shows every generated mix, uncapped and out of Playlists', () => {
+    const makeSongsLike = (id: string, name: string): Playlist => ({
+      id,
+      user_id: 'u1',
+      owner_username: 'u1',
+      name,
+      description: '',
+      is_public: false,
+      kind: 'system',
+      system_variant: 'songs_like_artist',
+      refreshable: false,
+      seed_artist_id: 'a1',
+      cover_url: '',
+      track_count: 25,
+      duration_sec: 1500,
+      created_at: '2026-05-04T00:00:00Z',
+      updated_at: '2026-05-04T00:00:00Z'
+    });
+    // Six generated mixes — the old carousel capped at 3; the dedicated row shows all.
+    const owned = Array.from({ length: 6 }, (_, i) => makeSongsLike(`sl${i}`, `Songs like ${i}`));
+    (createPlaylistsQuery as unknown as ReturnType<typeof vi.fn>).mockImplementation(
+      (kind?: string) =>
+        kind === 'system'
+          ? readable({ data: { owned, public: [] }, isPending: false, isError: false })
+          : readable({ data: emptyPlaylistsResponse, isPending: false, isError: false })
+    );
+    const { container } = render(Page);
+
+    for (let i = 0; i < 6; i++) {
+      expect(screen.getByText(`Songs like ${i}`)).toBeInTheDocument();
+    }
+    // The mixes live in the Songs-like row, not the Playlists carousel.
+    const playlistsSection = container.querySelector('[aria-label="Playlists"]') as HTMLElement;
+    expect(within(playlistsSection).queryByText('Songs like 0')).toBeNull();
+    const songsSection = container.querySelector('[aria-label="Songs like"]') as HTMLElement;
+    expect(within(songsSection).getByText('Songs like 0')).toBeInTheDocument();
+    // No placeholders once real mixes exist.
+    expect(within(songsSection).queryByTestId('playlist-placeholder-card')).toBeNull();
+  });
+
+  test('renders secondary system kinds (deep_cuts / new_for_you) in the Playlists row', () => {
     // Operator backflow 2026-06-01: web Home surfaces the 5 secondary
     // system kinds when generated. No placeholders for them — they
     // depend on library shape, so missing means "not enough data."