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/zh.ts

@@ -359,6 +359,7 @@ export const zhTranslation = {
     filterNoGenres: '未找到匹配流派',
     filterClear: '清除',
     bulkSelected: '已选 {{count}} 首',
+    clearSelection: '清除选择',
     bulkAddToPlaylist: '添加到播放列表',
     bulkRemoveFromPlaylist: '从播放列表移除',
     bulkClear: '取消选择',
@@ -895,6 +896,7 @@ export const zhTranslation = {
     titleBadge: '播放列表',
     refreshSuggestions: '新建议',
     addSong: '添加到播放列表',
+    addSelected: '添加所选',
     cacheOffline: '离线缓存播放列表',
     offlineCached: '播放列表已缓存',
     removeOffline: '从离线缓存中移除',