WIP: 一時コミット（sqlite-vecロールバック用の作業退避）

chore: comprehensive project restructuring and cleanup ...

- Moved backend source to src/backend
- Consolidated build artifacts to root target/
- Organized assets to resources/ and documentation to document/
- Updated README with detailed architecture and simplified onboarding
- Fixed minor bug in lib.rs database initialization
- Cleaned up obsolete Electron-era files and test scripts