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

fix: align all embedding dimensions to 384 for Gemma 3 300M consistency

feat: add datetime fields to MCP search results and include unit tests

chore: migrate to Tauri/Rust backend with SeaORM and add unit/integration tests ...

- Migrated Electron/Bun backend to Rust (Tauri v2).
- Integrated SeaORM with SQLite and sqlite-vec support.
- Implemented unit tests for LlamaClient and database initialization.
- Added integration tests for MCP handlers.
- Updated documentation (README.md, walkthrough, implementation plan).
- Improved dev environment with launch scripts and tasks.
- Followed user global rules for journaling and gitignore.