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/pages/Settings.tsx

@@ -4209,6 +4209,7 @@ function HomeCustomizer() {
     hero:            t('home.hero'),
     recent:          t('home.recent'),
     discover:        t('home.discover'),
+    discoverSongs:   t('home.discoverSongs'),
     discoverArtists: t('home.discoverArtists'),
     recentlyPlayed:  t('home.recentlyPlayed'),
     starred:         t('home.starred'),