chore: robust path resolution for production and installer compatibility ...

- Use Tauri Path Resolver for AppData (DB/Logs) and ResourceDir (DLLs)
- Automated vec0.dll syncing to resources/ during setup
- Improved sidecar path calculation for both dev and prod environments

feat: automate .env creation during setup for zero-config onboarding

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

feat: integrate llama-server sidecar with Gemma 3 300M support