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