docs: organize release instructions in README and development guide

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

chore: move src-tauri to src/backend for better logical structure

docs: comprehensive overhaul of README and documentation files for Tauri 2 and Gemma 3 integration