fix(collection): stage playlist track removals until Apply

[dylanjeffers]

Summary

• The bulk Remove button on the playlist edit view dispatched removeTrackFromPlaylist per selected id, which persisted each deletion to the backend immediately — bypassing the Apply/Discard gate. Because selection is keyed by track id, a track that repeats in a playlist expanded one selection into multiple removals, so Remove could wipe more than intended.
• Track removals are now staged in the edit-mode draft and only persisted when Apply is pressed (Undo/Discard restores them).

Changes

• PlaylistEditModeContext now owns removedTrackIds plus a batched undo/redo stack (stageRemoval/undoRemoval/redoRemoval, canUndoRemoval/canRedoRemoval). Staged removals count toward hasChanges and are persisted in apply() by filtering playlist_contents.track_ids. They reset atomically on enter/exit/discard/apply/conflict.
• TrackBulkActionsBar Remove stages only the selected ids (no saga, no backend write); Undo/Redo and Select-all exclude already-staged ids.
• EditAwareTracksTable hides staged-removed rows so they disappear immediately; Undo/Discard restores them in their original position.
• Removed the standalone TrackHistoryContext (its separate undo/redo stack could desync from the staged set across Discard/re-enter) and its provider.

Test plan

• Enter playlist edit mode, select one track, click Remove → only that track disappears; nothing is persisted yet.
• Press Apply → removal persists to the backend; toast reflects "Saved tracks".
• Press Discard (or Undo) → removed tracks reappear in their original positions; no backend write occurred.
• Remove several batches, then Undo/Redo repeatedly → staged set and table stay consistent.
• Re-enter edit mode after applying → Undo/Redo are disabled (no stale history).
• Select-all / Cmd-A excludes tracks already staged for removal.

🤖 Generated with Claude Code

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