feat: bulk multi-select + drag in PlaylistDetail, Favorites, and artist context menu (#157)

- PlaylistDetail: Ctrl/Cmd+Click enters select mode; bulk drag emits
  { type: 'songs', tracks } when ≥2 selected; filtered-view rows now
  also draggable as single songs
- Favorites songs: full multi-select system — Ctrl+Click, Shift+Click,
  header toggle-all checkbox, bulk-selected highlight, bulk drag to
  queue, bulk-bar with Add to Playlist + Clear
- ContextMenu: ArtistToPlaylistSubmenu resolves all artist album songs
  and forwards to AddToPlaylistSubmenu
- Locale: common.clearSelection + playlists.addSelected in all 7 locales

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/locales/de.ts

@@ -363,6 +363,7 @@ export const deTranslation = {
     filterNoGenres: 'Keine Genres gefunden',
     filterClear: 'Zurücksetzen',
     bulkSelected: '{{count}} ausgewählt',
+    clearSelection: 'Auswahl aufheben',
     bulkAddToPlaylist: 'Zur Playlist hinzufügen',
     bulkRemoveFromPlaylist: 'Aus Playlist entfernen',
     bulkClear: 'Auswahl aufheben',
@@ -901,6 +902,7 @@ export const deTranslation = {
     titleBadge: 'Playlist',
     refreshSuggestions: 'Neue Vorschläge',
     addSong: 'Zur Playlist hinzufügen',
+    addSelected: 'Ausgewählte hinzufügen',
     cacheOffline: 'Playlist offline speichern',
     offlineCached: 'Playlist gecacht',
     removeOffline: 'Aus Offline-Cache entfernen',