docs: mark Phase 12.3 (Export/Import) as complete

PLAN.md

@@ -781,10 +781,13 @@ audio-ui/
 - [x] Automation data (deep cloned to remove functions)
 - [x] Project metadata (name, description, duration, track count)
 
-#### 12.3 Project Export/Import
-- [ ] Export project as JSON (with audio files)
-- [ ] Import project from JSON
-- [ ] Project templates
+#### 12.3 Project Export/Import ✅
+- [x] Export project as JSON (with audio data embedded)
+- [x] Import project from JSON
+- [x] Export button per project in Projects dialog
+- [x] Import button in Projects dialog header
+- [x] Auto-generate new IDs on import to avoid conflicts
+- [ ] Project templates (future enhancement)
 
 #### 12.4 Project Settings ✅
 - [x] Sample rate (stored per project)