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/en.ts

@@ -364,6 +364,7 @@ export const enTranslation = {
     filterNoGenres: 'No genres match',
     filterClear: 'Clear',
     bulkSelected: '{{count}} selected',
+    clearSelection: 'Clear selection',
     bulkAddToPlaylist: 'Add to Playlist',
     bulkRemoveFromPlaylist: 'Remove from Playlist',
     bulkClear: 'Clear selection',
@@ -903,6 +904,7 @@ export const enTranslation = {
     titleBadge: 'Playlist',
     refreshSuggestions: 'New suggestions',
     addSong: 'Add to playlist',
+    addSelected: 'Add selected',
     cacheOffline: 'Cache playlist offline',
     offlineCached: 'Playlist cached',
     removeOffline: 'Remove from offline cache',