feat(18-02): add batch edit mode to track-details component
- showBatch() API for multi-track entry with merged field values - Three-state field model: keep/set/clear via dirty tracking in editValues - Confirmation dialog showing field changes before batch save - Progress bar with live counter wired to BatchWriteProgress events - Cancel button calling CancelBatchWrite during batch write - Results view with success/failure counts and expandable failure details - Batch cover art: pick, preview, or clear for all selected tracks - Post-save data refresh returning to updated summary view - Single-track show() path unchanged (batchMode = false)
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