v1.3.1 - Multi-Device Sync Fix + Performance

Multi-Device JSON Sync (Thanks Thomas!):
- JSON files now sync between devices
- Works without Markdown enabled
- No duplicate downloads anymore

Performance Improvements:
- Sync speed: 12-14s -> 2-3s
- First sync after MD export now fast
- JSON matches Markdown speed

+ Sync status UI, content MD import, debug tools