Fix 3 bug standalone: save project lỗi schema, export MIDI/WAV không hỏi nơi lưu

- Bug 1: serializeTracksList bỏ section item không có sectionId (do insertSectionAtPlayhead tạo) — tránh source_data {} làm schema oneOf fail
- Bug 2: export MIDI qua save-dialog + write file, toast kèm đường dẫn
- Bug 3: export mix WAV/MP3/OGG qua save-dialog + write file, toast kèm đường dẫn
- Marker tauri_bridge_ready: xóa khi window destroy + guard SF_PARENT_PID — tránh standalone treo 120s

app/main.py

@@ -17,6 +17,7 @@ from app.api.v1.user_config import router as user_config_router
 from app.api.v1.ai_proxy import router as ai_proxy_router
 from app.api.v1.ai_presets import router as ai_presets_router
 from app.api.v1.plugins import router as plugins_router
+from app.api.v1.files import router as files_router
 from app.api.v1.media import router as media_router
 from app.api.v1.system import router as system_router
 from app.api.v1.presets import router as presets_router
@@ -109,6 +110,7 @@ app.include_router(user_config_router, prefix="/api/v1/user", tags=["user_config
 app.include_router(ai_proxy_router, prefix="/api/v1/ai", tags=["ai"])
 app.include_router(ai_presets_router, prefix="/api/v1/ai", tags=["ai"])
 app.include_router(plugins_router, prefix="/api/v1/plugins", tags=["plugins"])
+app.include_router(files_router, prefix="/api/v1/files", tags=["files"])
 app.include_router(media_router, prefix="/api/v1/media", tags=["media"])
 app.include_router(system_router, prefix="/api/v1/system", tags=["system"])
 app.include_router(presets_router, prefix="/api/v1/presets", tags=["presets"])