Added highlight system types with strict color and sync status validation:
- HighlightColor type with 4 valid colors (yellow, orange, pink, blue)
- SyncStatus type for tracking sync state (pending, syncing, synced, error)
- BibleHighlight interface with full metadata support
- HighlightSyncQueueItem for offline sync queue management
- CrossReference interface for verse cross-referencing
Includes comprehensive test coverage validating type constraints.
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Co-Authored-By: Claude <noreply@anthropic.com>