feat: integrate the Windows titlebar menu

[wisdomqin]

Summary

• replace the default Windows menu bar with a compact titlebar menu beside the native window controls
• remove the visible titlebar spacer so the sidebar and conversation surface extend to the top edge
• keep a minimal draggable strip while preserving clickable application content
• synchronize the native controls and menu with Harness light and dark themes
• keep macOS window behavior unchanged

Validation

• npm run typecheck
• npm test (19 files, 99 tests)
• npm run build
• Windows x64 CI build and packaged Harness smoke test: https://github.com/dataelement/dsh-desktop/actions/runs/32225897069