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/styles/components.css

@@ -6187,7 +6187,7 @@ html.no-compositing .fs-lyrics-rail {
 
 .playlist-search-row {
   display: grid;
-  grid-template-columns: 36px 1fr auto 52px 28px;
+  grid-template-columns: 18px 36px 1fr auto 52px 28px;
   align-items: center;
   gap: var(--space-2);
   padding: 3px var(--space-1);
@@ -6199,6 +6199,18 @@ html.no-compositing .fs-lyrics-rail {
   background: var(--bg-hover);
 }
 
+.playlist-search-row--selected {
+  background: color-mix(in srgb, var(--accent) 10%, transparent);
+}
+
+.playlist-search-checkbox {
+  width: 14px;
+  height: 14px;
+  accent-color: var(--accent);
+  cursor: pointer;
+  flex-shrink: 0;
+}
+
 .playlist-search-thumb {
   width: 36px;
   height: 36px;