feat: add local Knowledge workspace

[yuting-ai]

Summary

• add local Knowledge libraries, folder sources, conversations, citations, and management UI
• index supported PDF, Markdown, text, DOCX, and HTML files with automatic source watching and progress SSE
• store chunk text and provenance in SQLite while using embedded Qdrant Local for vectors and scoped filtering
• bundle the multilingual 384-dimensional ONNX embedding model through Git LFS for offline runtime use
• document Knowledge behavior, supported formats, removal semantics, architecture, and source setup in both READMEs

Validation

• scripts/check.sh: 689 Python tests passed; Ruff, mypy, lock, and diff checks passed
• Desktop: 192 tests passed; typecheck, ESLint, and production build passed
• Python Sidecar: PyInstaller build and bundled smoke test passed