Delete old versions of features that are now in /reader/features/:
- offline-manager.ts (moved to features/)
- reading-speed-tracker.ts (moved to features/)
- panel-dock-system.ts (replaced by converted version in features/)
- navigator-panel.ts (replaced by converted version in features/)
All imports now use the /reader/features/ directory. This eliminates
duplication and makes it clear which files are the active ones.