Load file via ?vault=X&path=Y params using API. Auto-save 2s debounce with PUT /api/file/{vault}/save. Force save with Ctrl+S. Real AI integration: improve, fix-spelling, continue, summarize, translate, simplify, lengthen via /api/ai/* endpoints. AI status check + button states. Live markdown preview with marked.js CDN. Split view with draggable resize handle. Dirty/unsaved indicator. Vault+path in status bar. Word+char counter. All 5 zones preserved.