Add multi-select and batch actions to the album/playlist screen

[wenpeishao]

What

Adds QQ Music-style multi-select to the album and playlist detail screen. Long-press a track to enter selection mode; tap to toggle. A contextual app bar shows the selected count with select-all, and a bottom action bar performs the batch equivalents of the per-track actions:

• Play, add to queue, play next, add to next up
• Add to playlist (and remove from playlist inside playlists)
• Add / remove favourite

How

• Selection state is a screen-scoped Riverpod NotifierProvider.family (trackSelectionProvider, keyed by the album/playlist id). It's a plain manual provider, so no code generation is required.
• TrackListTile gains an opt-in selectionScope param. When set, tapping toggles selection and long-pressing enters selection mode (instead of opening the track menu), a selection indicator is drawn, and swipe is disabled. When null — every other use of the tile, e.g. playback history — the tile is unchanged.
• Batch queue actions build a PlayableSlice.simple(selectedTracks, source) and go through QueueService. Favourites reuse isFavoriteProvider(...).updateFavorite(...). Add / remove to playlist reuses showPlaylistActionsMenu(items:, parentPlaylist:).
• New English strings (selectAll, deselectAll, itemsSelected) are added to app_en.arb; everything else reuses existing keys.

Notes

• Scoped to the album/playlist detail screen for a focused first PR. The shared TrackListTile change makes extending this to the Songs tab a small follow-up.
• Download / delete batch actions are intentionally left for a follow-up.
• Passes flutter analyze with no new errors.