feat(home): add Discover Songs rail to Mainstage (#301)

Reuses the SongRail component from the Tracks hub — 18 random songs
fetched in parallel with the other Home queries. No reroll button.
Click-to-play uses enqueueAndPlay so the existing queue stays intact.

New homeStore section id 'discoverSongs' inserted after 'discover'
in DEFAULT_HOME_SECTIONS. onRehydrateStorage now appends any newly
introduced sections to a previously persisted layout — existing users
get the rail without a manual Reset. Settings → Personalisation
Home-Customizer surfaces it automatically via SECTION_LABELS.

i18n key home.discoverSongs added to all 8 locales.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/locales/zh.ts

@@ -41,6 +41,7 @@ export const zhTranslation = {
     mostPlayed: '最常播放',
     recentlyPlayed: '最近播放',
     discover: '发现',
+    discoverSongs: '发现曲目',
     loadMore: '加载更多',
     discoverMore: '发现更多',
     discoverArtists: '发现艺术家',